Default to PostgreSQL 11 in Omnibus

Summary

PostgreSQL 11 was added as an opt-in version of PostgreSQL in GitLab 12.8. In 12.10 we will make it the default version for new installs and upgrades.

Requirements

• For new installs of Omnibus GitLab, PostgreSQL 11 is used by default
• Users can opt to use PostgreSQL 9.6 or 10 during a new install
• During upgrades, the database is automatically upgraded to PostgreSQL 11 (from 9.6 or 10)
• During upgrades, users have the option to stay on PostgreSQL 9.6 or 10
• During upgrades, if a user opts to stay on an older version of PostgreSQL they see a deprecation notice warning that PostgreSQL 11 will become the minimum required version in GitLab 13.0