Drop support for elasticsearch 5.6

Currently, GitLab supports elasticsearch v5.6 (the only version supporting both parent/child and join_field) and v6.x . It does not support elasticsearch v7: #12641 (closed)

In gitlab-elasticsearch-indexer!34 (comment 226929111) , we learned that the libraries we're using only formally support one ES version at a time anyway - in the case of the golang indexer, that's currently 5.6, but it issues deprecation warnings in the logs a lot when run against ES6. There are no other negative consequences that we can see.

Since we've supported ES6 for a number of releases now, and since supporting 5.6, 6.x and 7.x would be a major strain, I propose we remove support for ES5.6, leaving 6.x the only supported version for now. @phikai seemed supportive of this route in the other issue. This may also help us to update our elasticsearch-ruby dependency.

Before adding ES7 support, we may need to revisit how we support multiple major versions of elasticsearch, but it doesn't need to block this issue specifically.