feat: prepare ES8 support by fixing 'cannot import RequestsHttpConnection'

[julian-risch]

Related Issues

• As part of the support of elasticsearch 8, this PR is related to Add support for Elasticsearch 8 #2810

Proposed Changes:

• Pin elasticsearch client to version 8
• Change ElasticSearchDocumentStore so that it works with version 8
• Add an ElasticSearchDocumentStore7 class that inherits from ElasticSearchDocumentStore and works with version 7
• Show deprecation warning if somebody uses ElasticSearchDocumentStore with version 7 and make sure it works nevertheless
• Add tests for both version 7 and version 8
• Unpin elasticsearch client version and decide on whether to keep elasticsearch and elastic_transport in standard Haystack dependencies

How did you test it?

Notes for the reviewer

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• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added tests that demonstrate the correct behavior of the change
• I've used the conventional commit convention for my PR title
• I documented my code
• I ran pre-commit hooks and fixed any issue

[nickchomey]

FYI, ES 8.4 changed the ANN mechanism to use the _search endpoint. So, it might be worth pinning to that instead.

https://www.elastic.co/guide/en/elasticsearch/reference/current/release-highlights.html#integrate_ann_into_search_endpoint