Update Postgres to 12.7
Created by: david-sandy
This is the newly created PR for Postgres updates. It renames the "postgres-12.x" references with "postgres-12".
It will supersede: https://github.com/sourcegraph/sourcegraph/pull/30917 https://github.com/sourcegraph/sourcegraph/pull/31443
Test plan
Test locally, dry-run through CI. More?