Geo: Test and document upgrading tracking database with Patroni to Postgres 12

Problem

While the upgrade of that cluster should be fairly standard, I don't think I'm clear what other steps are needed. For example, pausing or shutting down Geo.

One thing jumps at me as different for the tracker database: it's called gitlabhq_geo_production and not gitlabhq_production. I don't know if there's potential for this to cause issues. I might be confusing it with a limitation of repmgr.

It should be clear to all how to upgrade to PG12. It should be documented.

Proposal

• Test and document upgrading tracking database with Patroni to Postgres 12