Add PostgreSQL 12 to CI testing
This issue is the first step in a phased approach to introduce support for PG12.
The scope of this issue is to add PG12 to CI testing so that we can start testing compatibility and upgrades. Issues identified as a result of adding PG12 to CI testing should be addressed in %12.10.
Per @twk3's comment in &2184 (comment 265916460), when we announce support we will need to call out https://docs.gitlab.com/omnibus/settings/database.html#backup-and-restore-a-non-packaged-postgresql-database which for postgres 12 will need users to bring their own pgsql in order to have backup/restore working if they are doing db backups from an Omnibus instance.
Out of Scope
This issue does not cover packaging PG12 in Omnibus. See the parent epic for the timeline for packaging PG12.