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