CI: Drop testing for Postgres 12

What does this MR do and why?

This MR removes all PG12-related CI configuration as PG12 was dropped in GitLab 16.0 release.

GitLab 16.0 release announcement:

PostgreSQL 12 is no longer supported. The minimum required version is PostgreSQL 13.

Follow-up of !135358 (merged).

Contributes to #425471 (closed).

How to verify locally

$ rg pg12
changelogs/archive-13.md
267:- Fix issuable search optimization in PG12. !61880

lib/gitlab/database/reindexing/reindex_concurrently.rb
6:      # This is a >= PG12 reindexing strategy based on `REINDEX CONCURRENTLY`
28:          # In PG12, this has been fixed in https://gitlab.com/postgres/postgres/-/commit/b17ff07aa3eb142d2cde2ea00e4a4e8f63686f96.

CHANGELOG.md
4833:- [Add 16.0 breaking change removal notice for PG12](gitlab-org/gitlab@d94d61a516b35e9860efe207241a29afd017d28a) ([merge request](gitlab-org/gitlab!120343))

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