Rename merge_requests.allow_maintainer_to_push -> merge_requests.allow_collaboration
https://gitlab.com/gitlab-org/gitlab-ce/issues/47566 noted that there was a problem with this migration at GitLab.com's scale. We have https://gitlab.com/gitlab-org/gitlab-ce/issues/47591 to clarify the docs, and once that's done, we should actually rename this column.