Update GitLab Migration to use /metadata endpoint instead of /version

As pointed out by @xanf /version endpoint is getting deprecated in favour of /metadata #366287 (closed)

In GitLab migration we use /version endpoint to determine what kind of data we can import and what kind of limitations a particular version has.

We should update it to use /metadata

Usage example: https://gitlab.com/gitlab-org/gitlab/blob/5ccb5a0ece208fc74da096274357b112bfb529f2/lib/bulk_imports/clients/http.rb#L57-65

https://docs.gitlab.com/ee/api/metadata.html says this endpoint was introduced in 15.1, so we should make sure previous versions of GitLab still use /version in order to provide compatibility.