Test if ES Client version 7.x is compatible with ES 6.x server

Related to #327560 (closed).

Given that our compatibility with Elasticsearch version 6.x is conflicting with out ability to use Opensearch 1.2.x #327560 (comment 829496980) we want to test if deprecating ES client 6.x and having our self hosted customers upgrade to ES Client 7.x are still able to use ES backend version 6.x.

If an ES 7.x client can still integrate with an ES 6.x server then we will have less cause for delaying our deprecation of 6.x on the GitLab side and be able to integrate with Opensearch more quickly.