Update GDK to use PostgreSQL v12.9

What does this Merge Request do and why?

We're upgrading GitLab.com from 12.7 to 12.9 soon (https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14582) to address a subtransaction performance issue.

Omnibus: omnibus-gitlab!5728 (merged)

Announcement: https://www.postgresql.org/about/news/postgresql-141-135-129-1114-1019-and-9624-released-2349/

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.