www/gitlab-ce: Upgrade to next major version 12.1

It requires PostgreSQL version 10.7, so make sure you upgrade PostgreSQL first.
I recommand to go to version 11 as described in UPDATING entry 20190829.

MySQL support was now completely removed upstream.

Changelogs:
https://about.gitlab.com/2019/07/22/gitlab-12-1-released/
https://about.gitlab.com/2019/07/24/gitlab-12-1-1-released/
https://about.gitlab.com/2019/07/29/security-release-gitlab-12-dot-1-dot-2-released/
https://about.gitlab.com/2019/07/31/gitlab-12-1-3-released/
https://about.gitlab.com/2019/08/06/gitlab-12-1-4-released/
https://about.gitlab.com/2019/08/12/critical-security-release-gitlab-12-dot-1-dot-6-released/
https://about.gitlab.com/2019/08/29/security-release-gitlab-12-dot-2-dot-3-released/