Update replicate-geo-database to support PostgreSQL 12

Context

Follow-up from verification testing of PG12 with Geo installs:

After some manual adjustments, I verified replication is working for the various data types we've listed in the description. At the moment, deploying Geo with PostgreSQL 12 is not recommended.

This is what I think is needed to support PG12 with a fresh Geo install:

• Pre 14.0 release (13.3 if possible, but might have to be 13.4 at this point)
  • Modify gitlab-ctl replicate-geo-database to detect if PG12 is enabled. If enabled, instead of creating recovery.conf, the script writes the settings to postgresql.conf (except standby_mode), and creates files for recovery.signal and standby.signal.

Proposal

• Modify gitlab-ctl replicate-geo-database to detect if PG12 is enabled.
  • If enabled, instead of creating recovery.conf, the script writes the settings to postgresql.conf (except standby_mode), and creates standby.signal file.
• Re-verify Geo installation with PostgreSQL 12