Review upgrade to Elasticsearch 9

[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