Doc change request, opensearch incompatible parameter

## Expected Behavior

Documentation should contain that elasticsearch_version parameter has to be unset.

## Current Behavior

If parameter elasticsearch_version was set previously due to problem with probing move to opensearch will fail.
In the graylog server log it can be seen that opensearch backend is recognized
`[SearchDbPreflightCheck] Connected to (Elastic/Open)Search version `
But the cluster will be set to Elastic
`[ElasticsearchVersionProvider] Elasticsearch version set to Elasticsearch:7.0.0 - disabling version probe.`

System will startup, and it will see Opensearch status, indicies, etc. but it will fail to read or write data with
`ElasticsearchException{message=ElasticsearchException[An error occurred: ]; nested: IOException[Unable to parse response body for Response{requestLine=POST /_bulk?timeout=1m HTTP/1.1, host=http://127.0.0.1:9200, response=HTTP/1.1 200 OK}]; nested: NullPointerException;, errorDetails=[]}
Suppressed: org.graylog.shaded.elasticsearch7.org.elasticsearch.client.ResponseException: method [POST], host [http://127.0.0.1:9200], URI [/_msearch?typed_keys=true], status line [HTTP/1.1 400 Bad Request]
{"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"key [types] is not supported in the metadata section"}],"type":"illegal_argument_exception","reason":"key [types] is not supported in the metadata section"},"status":400}
`

## Possible Solution

Just add step to documentation to check this parameter is unset.

## Steps to Reproduce (for bugs)

1. set parameter elasticsearch_version with opensearch backend.