Upgrade to the new Elastic Java client from currently deprecated RestHighLevelClient, add proper Elasticsearch 8/9 support

[fludo]

The Java High Level REST Client is deprecated in Elastic 7.15.0, in favour of the Java API Client: https://www.elastic.co/guide/en/elasticsearch/client/java-rest/current/java-rest-high.html

Despite compatibility mode allowing the 7.17 High Level Rest Client to work with Elasticsearch 8.x, it is not compatible with ElasticSearch 9.

Guide for the upgrade: https://www.elastic.co/guide/en/elasticsearch/client/java-api-client/8.18/migrate-hlrc.html

This will fix:
#847
#828

[yeikel]

This will also fix #892