Throw error if ES version is not supported

[TuanaCelik]

There was a breaking change with Elasticsearch 7.6 as mentioned in this issue: #5066 which means Haystack doesn't support the vector similarity function.
I suggest we start:

• Throw an error if a version unsupported by Haystack is used. @julian-risch suggested somewhere around here and that client.info() might return a dict containing also the server version

  haystack/haystack/document_stores/elasticsearch.py

  Line 265 in a9a49e2

  status = client.ping()

• Listing the document store versions we support with Haystack if possible

[jscheel]

I think we can easily support this by storing the ES cluster version internally in the document store, then tweaking the painless script args in _get_vector_similarity_query based on that. Specifically, we would just need to look at the version number to determine if the script should look like this:

"source": f"{similarity_fn_name}(params.query_vector,'{self.embedding_field}') + 1000"

or like this:

 "source": f"{similarity_fn_name}(params.query_vector,doc['{self.embedding_field}']) + 1000"

There doesn't seem to be support for getting the cluster version number in painless, so it would be maintained in the store itself. I can put together a PR if you are open to it.

[ZanSara]

@jscheel please feel free to open a PR!

[jscheel]

Awesome, will do. Headed out of the country for a few weeks, but will take a look when I get back.

…

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[jscheel]

@bogdankostic awesome! Thanks for taking care of this! Sorry I haven't been able to look at it. Just got back into the swing of things after my trip.