Prior to this commit, Spring Boot would auto-configure both
Elasticsearch variants: `RestClient` ("Low Level" client) and
`RestHighLevelClient` ("High Level" client).
Since one can be derived from the other, this would create complex and
unclear situations depending on what developers provided with their
configuration.
`RestHighLevelClient` is mostly for actual use of the Elasticsearch API,
with support for specific methods and (de)serialization. On the other
hand, `RestClient` is merely wrapping the Apache HTTP client for
load-balancing support and low level HTTP features.
This commit completely removes the support for `RestClient` in Spring
Boot and now requires the presence of the
`org.elasticsearch.client:elasticsearch-rest-high-level-client`
dependency for REST client support with Elasticsearch.
Closes gh-22358
1d73d4eda7