Geo: Validate PG 11 manual upgrade on 12.10

Setup

• I already have a Geo setup with separate databases from testing in #5233 (closed). They are on GitLab 12.10-pre built from !4115 (merged). The DBs are on PG 10.12. See my notes: https://docs.google.com/document/d/1VaZF4EM5_xRcUaZvQdWlV_C7TWW__rM2qhldf3X8jb0/edit#
• Upgrade to a GitLab 12.10-pre package that includes !4091 (merged). The package built from built from !4115 (merged) did not include !4091 (merged) at the time of writing. I believe we should wait for a package that includes both of the above MRs. Either when !4115 (merged) is merged to master, or we could rebase it and build another package.
• Double check that all 3 DB nodes are still on PG 10.X
• Double check that everything is working: Run GitLab QA Geo specs (excluding SSH tests)

Test

• Manually upgrade PostgreSQL on all 3 DB nodes with https://docs.gitlab.com/omnibus/settings/database.html#upgrading-a-geo-instance, which now includes !4072 (merged)
• Run GitLab QA Geo specs (excluding SSH tests)

Expectation

• PostgreSQL successfully upgrades on all 3 DB nodes to PG 11.X

• QA Geo specs pass

• Open follow up issues, if any => #5271 (closed)

If you want more detail, I took some notes in https://docs.google.com/document/d/1VaZF4EM5_xRcUaZvQdWlV_C7TWW__rM2qhldf3X8jb0/edit# (internal link)