Verify Geo installation with PostgreSQL 12

After PostgreSQL 12 is bundled in omnibus-gitlab, we need to ensure that fresh installations of GEO are functional with PostgreSQL 12.

We should verify the following scenarios

1. Single node primary, and single node secondary instances
2. Separate database nodes on both primary and secondary
3. [ ] On the secondary, running with separate postgresql, and geo-postgresql nodes

1. Single node

Tests:

• Repository
• Mirrored repository
• Wiki
• Uploads (wiki)
• LFS objects
• Job artifacts

2. Separate database nodes

Tests: https://gitlab.com/gitlab-org/quality/ci/no-downtime-upgrades/-/pipelines/176798729

3. All databases separated

Will test after #5575 (closed) has merged.