Set policy for minimum supported GitLab version

At present there are various conditionals in the code base that check the GitLab version and handle any discrepancies in the API between these versions. This currently includes checks for 9.2.2, 9.2.3, 10.5.0, 11.2.

While I think it's a good idea to support the latest and the previous major version of GitLab, I think we should aim to remove dead code to handle versions no longer supported by GitLab upstream.