Add postgres removal warning for PG12 in GitLab 16.0

Postgres 12 support is being removed in GitLab 16.0 https://about.gitlab.com/handbook/engineering/development/enablement/data_stores/database/postgresql-upgrade-cadence.html

• Update the min supported GitLab version in the rails codebase
  • See gitlab!63471 (merged) for the previous time this was updated
  • We received feedback in the past that UI GitLab admins (not sysadmins) find this warning annoying if we announce too early. Which is why this notice update goes in with the removal of support. For sysadmins we display notices during upgrades via #7393 (closed)
• Set the min PG version check version in omnibus like !5348 (merged)
• Update the deprecation messages used in #7393 (closed) to now be in past tense, as the support is now dropped