Auto-configure Elasticsearch's new Rest5Client rather than the legacy RestClient

[snicoll]

The upgrade made it so that RestClient isn't automatically on the classpath. However ElasticsearchClientAutoConfiguration is guarded by the fact that such a bean has been configured. We'd need to see if there is an alternative option that doesn't need the extra library.

Removing org.elasticsearch.client:elasticsearch-rest-client from spring-boot-testcontainers can reproduce that issue.

We also moved our test altogether to ES9 server. Perhaps we still want to test against ES8, although it needs additional settings to work, see elastic/elasticsearch-ruby#2665

[wilkinsona]

Rest5Client appears to be the replacement. It's part of elasticsearch-java so we already have it on the classpath. We should remove org.elasticsearch.client:elasticsearch-rest-client from spring-boot-starter-data-elasticsearch as part of switching to the new client.

[wilkinsona]

We also moved our test altogether to ES9 server

Tests that use TestImage.container(ElasticsearchContainer.class) are still using Elasticsearch 7.

[wilkinsona]

I have something for this, but I'd like to wait and see if there are any improvements to the configurability of the new client before proceeding.