Consolidate Elasticsearch configuration properties

Previously, a number of Elasticsearch properties were duplicated
across the spring.elasticsearch.rest and
spring.data.elasticsearch.client.reactive prefixes for configuring
the blocking REST client provided by Elasticsearch and the reactive
client provided by Spring Data respectively. This could cause
problems when using the Elasticsearch REST client configured with
a custom spring.elasticsearch.rest.uris. If Spring WebFlux (to make
use of WebClient) and Spring Data Elasticsearch were on the classpath,
the reactive Elasticsearch Client would be autoconfigured but it
would use the default value of its analogous
spring.data.elasticsearch.client.reactive.endpoints property. It
would be unable to connect, causing a startup failure.

This commit consoliates the configuration properties where possible.
Each setting that is common across the two clients is now configured
using a single, shared spring.elasticsearch property. Each setting
that is specific to the blocked REST client or the WebClient-based
reactive client now have prefixes of spring.elasticsearch.restclient
and spring.elasticsearch.webclient respectively.

The old properties beneath spring.elasticsearch.rest and
spring.data.elasticsearch.client.reactive have been deprecated. If a
any deprecated property is set, all of the new properties are
ignored. In other words, to migrate to the new properties, each usage
of a now-deprecated property must be updated to use its new
replacement instead.

Closes gh-23106

spring-boot-project/spring-boot-docs/src/docs/asciidoc/data/nosql.adoc

@@ -261,7 +261,7 @@ If you add your own `@Bean` of type `SolrClient`, it replaces the default.
 [[data.nosql.elasticsearch]]
 === Elasticsearch
 https://www.elastic.co/products/elasticsearch[Elasticsearch] is an open source, distributed, RESTful search and analytics engine.
-Spring Boot offers basic auto-configuration for Elasticsearch.
+Spring Boot offers basic auto-configuration for Elasticsearch clients.
 
 Spring Boot supports several clients:
 
@@ -276,34 +276,36 @@ Spring Boot provides a dedicated "`Starter`", `spring-boot-starter-data-elastics
 ==== Connecting to Elasticsearch using REST clients
 Elasticsearch ships https://www.elastic.co/guide/en/elasticsearch/client/java-rest/current/index.html[two different REST clients] that you can use to query a cluster: the "Low Level" client and the "High Level" client.
 Spring Boot provides support for the "High Level" client, which ships with `org.elasticsearch.client:elasticsearch-rest-high-level-client`.
-
-If you have this dependency on the classpath, Spring Boot will auto-configure and register a `RestHighLevelClient` bean that by default targets `http://localhost:9200`.
-You can further tune how `RestHighLevelClient` is configured, as shown in the following example:
+Additionally, Spring Boot provides support for a reactive client, based on Spring Framework's `WebClient`, that ships with `org.springframework.data:spring-data-elasticsearch`.
+By default, the clients will target `http://localhost:9200`.
+You can use `spring.elasticsearch.*` properties to further tune how the clients are configured, as shown in the following example:
 
 [source,yaml,indent=0,subs="verbatim",configprops,configblocks]
 ----
 	spring:
 	  elasticsearch:
-	    rest:
-	      uris: "https://search.example.com:9200"
-	      read-timeout: "10s"
-	      username: "user"
-	      password: "secret"
+	    uris: "https://search.example.com:9200"
+	    socket-timeout: "10s"
+	    username: "user"
+	    password: "secret"
 ----
 
-You can also register an arbitrary number of beans that implement `RestClientBuilderCustomizer` for more advanced customizations.
-To take full control over the registration, define a `RestClientBuilder` bean.
+[[data.nosql.elasticsearch.connecting-using-rest.restclient]]
+===== Connecting to Elasticsearch using RestHighLevelClient
+If you have `elasticsearch-rest-high-level-client` on the classpath, Spring Boot will auto-configure and register a `RestHighLevelClient` bean.
+In addition to the properties described previously, to fine-tune the `RestHighLevelClient`, you can register an arbitrary number of beans that implement `RestClientBuilderCustomizer` for more advanced customizations.
+To take full control over its registration, define a `RestClientBuilder` bean.
 
 TIP: If your application needs access to a "Low Level" `RestClient`, you can get it by calling `client.getLowLevelClient()` on the auto-configured `RestHighLevelClient`.
 
-Additionally, if `elasticsearch-rest-client-sniffer` is on the classpath, a `Sniffer` is auto-configured to automatically discover nodes from a running Elasticsearch cluster and set them to the `RestHighLevelClient` bean.
+Additionally, if `elasticsearch-rest-client-sniffer` is on the classpath, a `Sniffer` is auto-configured to automatically discover nodes from a running Elasticsearch cluster and set them on the `RestHighLevelClient` bean.
 You can further tune how `Sniffer` is configured, as shown in the following example:
 
 [source,yaml,indent=0,subs="verbatim",configprops,configblocks]
 ----
 	spring:
 	  elasticsearch:
-	    rest:
+	    restclient:
 	      sniffer:
 	        interval: 10m
 	        delay-after-failure: 30s
@@ -311,31 +313,23 @@ You can further tune how `Sniffer` is configured, as shown in the following exam
 
 
 
-[[data.nosql.elasticsearch.connecting-using-reactive-rest]]
-==== Connecting to Elasticsearch using Reactive REST clients
+[[data.nosql.elasticsearch.connecting-using-rest.webclient]]
+===== Connecting to Elasticsearch using ReactiveElasticsearchClient
 {spring-data-elasticsearch}[Spring Data Elasticsearch] ships `ReactiveElasticsearchClient` for querying Elasticsearch instances in a reactive fashion.
 It is built on top of WebFlux's `WebClient`, so both `spring-boot-starter-elasticsearch` and `spring-boot-starter-webflux` dependencies are useful to enable this support.
 
-By default, Spring Boot will auto-configure and register a `ReactiveElasticsearchClient`
-bean that targets `http://localhost:9200`.
-You can further tune how it is configured, as shown in the following example:
+By default, Spring Boot will auto-configure and register a `ReactiveElasticsearchClient`.
+In addition to the properties described previously, the `spring.elasticsearch.webclient.*` properties can be used to configure reactive-specific settings, as shown in the following example:
 
 [source,yaml,indent=0,subs="verbatim",configprops,configblocks]
 ----
 	spring:
-	  data:
-	    elasticsearch:
-	      client:
-	        reactive:
-	          endpoints: "search.example.com:9200"
-	          use-ssl: true
-	          socket-timeout: "10s"
-	          username: "user"
-	          password: "secret"
+	  elasticsearch:
+	    webclient:
+	      max-in-memory-size: 1MB
 ----
 
-If the configuration properties are not enough and you'd like to fully control the client
-configuration, you can register a custom `ClientConfiguration` bean.
+If the `spring.elasticsearch.*` and `spring.elasticsearch.webclient.*` configuration properties are not enough and you'd like to fully control the client configuration, you can register a custom `ClientConfiguration` bean.
 
 
 
@@ -352,7 +346,7 @@ as shown in the following example:
 include::{docs-java}/features/nosql/elasticsearch/connectingusingspringdata/MyBean.java[]
 ----
 
-In the presence of `spring-data-elasticsearch` and the required dependencies for using a `WebClient` (typically `spring-boot-starter-webflux`), Spring Boot can also auto-configure a <<features#data.nosql.elasticsearch.connecting-using-reactive-rest,ReactiveElasticsearchClient>> and a `ReactiveElasticsearchTemplate` as beans.
+In the presence of `spring-data-elasticsearch` and the required dependencies for using a `WebClient` (typically `spring-boot-starter-webflux`), Spring Boot can also auto-configure a <<features#data.nosql.elasticsearch.connecting-using-rest.webclient,ReactiveElasticsearchClient>> and a `ReactiveElasticsearchTemplate` as beans.
 They are the reactive equivalent of the other REST clients.