Add PostgreSQL 14 as opt-in for fresh installs

Note the migrations section of https://www.postgresql.org/docs/14/release-14.html#id-1.11.6.12.4

This is a followon to #7396 (closed)

Previous work was here: !5390 (merged)

Acceptance criteria

• Binary is shipped in omnibus and usable for performing backups/restores against a remote PG14
• Single node fresh installs of GitLab can opt to use PG14
• Patroni HA does not run into obvious errors during setup, deferred to #7766 (closed)
• Geo setups do not run into obvious errors during setup, deferred to #7765 (closed)

Followup issues will be made for:

• Verify PG-HA (failover etc)
• Verify GEO (failover etc)
• Supporting upgrades as a whole