Test EE -> CE migration path

We've seen numerous issues (e.g. https://gitlab.com/gitlab-org/gitlab-ce/issues/46775) where a backported CE feature doesn't have the right database migrations when migrating from EE to CE.

We should test that EE releases can be upgraded to the next CE version seamlessly. We may want to do this in GitLab QA and/or in the EE code base (as done in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5673).

/cc: @meks, @godfat, @DouweM, @tiagonbotelho