Set PostgreSQL 13 as the minimum supported version

What does this MR do and why?

Set PostgreSQL 13 as the minimum supported version in GitLab 16.0

This will cause a warning to be output to the logs if you are running an older version, as well as showing a notice in the admin section.

Screenshots or screen recordings

How to set up and validate locally

1. Set up a PG 12 instance
   1. Download and install omnibus-gitlab package 15.11.0
   2. Follow https://docs.gitlab.com/ee/administration/postgresql/standalone.html, but in addition to the config mentioned there, add postgresql['version'] = 12 also to /etc/gitlab/gitlab.rb before running gitlab-ctl reconfigure
2. Configure a GitLab instance using the package from this MR (or GDK, whichever works) and configure it to use the PG 12 instance setup above. If using package, follow https://docs.gitlab.com/omnibus/settings/database.html#using-a-non-packaged-postgresql-database-management-server.
3. Run a rake task using the DB, and see the warning in the terminal
4. Open Admin page in the GitLab settings and see the warning there also.

MR acceptance checklist

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