Configure Spring Data Couchbase explicitly

This commit configures Spring Data Couchbase explicitly rather than
relying on the abstract configuration class. This has the advantage of
simplifying the auto-configuration and let it us proxy-free
configuration classes.

Spring Boot no longer uses or interacts with CouchbaseConfigurer. Users
relying on that to teach Spring Boot which components to use should
rely on `@Primary` flag instead in case of multiple beans of the same
type.

`CouchbaseConfiguration` is no longer public as extending from it is
no longer necessary. If the `CouchbaseEnvironment` has to be
customized, a `CouchbaseEnvironmentBuilderCustomizer` bean can be
registered to tune the auto-configured environment.

Closes gh-20533

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -4485,7 +4485,6 @@ Generally, you provide the bootstrap hosts, bucket name, and password, as shown
 ----
 
 TIP: You need to provide _at least_ the bootstrap host(s), in which case the bucket name is `default` and the password is an empty String.
-Alternatively, you can define your own `org.springframework.data.couchbase.config.CouchbaseConfigurer` `@Bean` to take control over the whole configuration.
 
 It is also possible to customize some of the `CouchbaseEnvironment` settings.
 For instance, the following configuration changes the timeout to use to open a new `Bucket` and enables SSL support:
@@ -4497,7 +4496,8 @@ For instance, the following configuration changes the timeout to use to open a n
 	spring.couchbase.env.ssl.key-store-password=secret
 ----
 
-Check the `spring.couchbase.env.*` properties for more details.
+TIP: Check the `spring.couchbase.env.*` properties for more details.
+To take more control, one or more `CouchbaseEnvironmentBuilderCustomizer` beans can be used.