Default to PostgreSQL 12 for upgrades

In %13.6, PostgreSQL 12 became the default version for new installs, with the option to stay on PostgreSQL 11. In %13.7, it is due to become the default version for upgrades.

Requirements

• During upgrades, the database is automatically upgraded to PostgreSQL 12 (from 11)
• During upgrades, users have the option to stay on PostgreSQL 11
• During upgrades, if a user opts to stay on PG11 they see a deprecation notice warning that PostgreSQL 12 will become the minimum required version in GitLab 14.0
• Installs with Geo may need to be excluded from auto upgrades due to some manual steps that are required. See #5233 (closed) for context.