Default to PostgreSQL 12 in Omnibus for new installs

Summary

In 13.6 we will make PG12 the default version for new installs and upgrades.

Requirements

• 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)