Add Elasticsearch 8.0 support

Since Elasticsearch 8.0 is officially released, I think it's time to consider adding es8 connector support.

The High Level REST Client we used for connection is marked deprecated in es 7.15.0. Maybe we can migrate to use the new Java API Client at this time.

Elasticsearch8.0 release note: https://www.elastic.co/guide/en/elasticsearch/reference/8.0/release-notes-8.0.0.html

release highlights: https://www.elastic.co/guide/en/elasticsearch/reference/8.0/release-highlights.html

REST API compatibility: https://www.elastic.co/guide/en/elasticsearch/reference/8.0/rest-api-compatibility.html