Update Elasticsearch client to version 9.x

[l-trotta]

The Elasticsearch java client released a new major version 9.0, introducing a number of breaking changes that will affect this and other spring projects.
I have a draft ready with the minimal changes to make spring-data-elasticsearch work with version 9.0.1 of the client; I'd like to open a PR where we could discuss those and other possible changes, which could be part of the next major release of spring-data-elasticsearch.

Disclaimer: I am one of the maintainer of https://github.com/elastic/elasticsearch-java