Make ElasticsearchDocumentStore compatible with ES8

[masci]

Now that the Elasticsearch client is not part anymore of the core installation, we can introduce a separate extras to install the version 8 of the Python client.

Figure out whether or not the existing ElasticsearchDocumenStore can be made compatible with both clients with a few tweaks, or it's better to introduce a new document store class solely compatible with 8.x

[bogdankostic]

Support for Elasticsearch 8 has been added in the last weeks. ElasticsearchDocumentStore running on Elasticsearch 8 can be found in haystack/document_stores/elasticsearch/es8.py.