Default to PostgreSQL 12 in Omnibus for new installs
In 13.6 we will make PG12 the default version for new installs and upgrades.
For new installs of Omnibus GitLab, PostgreSQL 12 is used by default
Users can opt to use PostgreSQL 11 during a new install
Upgrades of existing deployments will stay on PostgreSQL 11 unless the user manually upgrades the PostgreSQL version
Issue used when defaulting to PG 11: #5092 (closed)