Single node upgrades to PostgreSQL 13.x

The Omnibus package should install PostgreSQL 11 by default. But, running gitlab-ctl pg-upgrade -V 13 manually should upgrade the database to the bundled PostgreSQL 13.

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 12.X to 13.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 13.X in separate issues.

Issue used when adding PG 12: #5495 (closed)