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)

Edited Oct 15, 2020 by Larissa Lane