Single node upgrades to PostgreSQL 16.x

The Omnibus package should install PostgreSQL 14 by default after Add support for PostgreSQL 14 (&9065 - closed) • Unassigned is closed. But, running gitlab-ctl pg-upgrade -V 16 manually should upgrade the database to the bundled PostgreSQL 16.

This should work on single node installations, and installations using a separate database node with the omnibus-gitlab provided PostgreSQL. We will handle GEO and PG HA installations in separate issues.

The gitlab-ctl pg-upgrade command uses the upstream pg_upgrade command behind the scenes to upgrade the database files to work with a newer PostgreSQL version.

We need to verify that it is safe to upgrade from 14.X to 16.X using pg_upgrade using a single node installation.

Once we've verified here that manually upgraded databases are functional, we will handle automatic upgrades to PostgreSQL 16.X in separate issues.

Issue used when adding PG 13: #6129 (closed)