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

src/checkstyle/checkstyle-suppressions.xml

@@ -54,4 +54,7 @@
 	<suppress files="BsdDomainSocket" checks="FinalClass" message="SockaddrUn" />
 	<suppress files="StringSequence" checks="SpringMethodVisibility" message="isEmpty"/>
 	<suppress files="ValidatorPropertiesWithDefaultValues\.java" checks="SpringMethodVisibility" />
+	<suppress files="DeprecatedElasticsearchRestClientProperties\.java" checks="SpringMethodVisibility" />
+	<suppress files="DeprecatedElasticsearchRestClientProperties\.java" checks="SpringMethodVisibility" />
+	<suppress files="DeprecatedReactiveElasticsearchRestClientProperties\.java" checks="SpringMethodVisibility" />
 </suppressions>