Use pg version 12 in troubleshooting PG

What does this Merge Request do and why?

Use Homebrew's postgresql@12 instead of 11 in troubleshooting PostgreSQL.

cf. !2624 (merged)

Updates !451 (merged), !898 (merged), and !1520 (merged)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• [n/a] Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• [n/a] gdk doctor test added, if needed.
• [n/a] Add the ~highlight label if this MR should be included in the CHANGELOG.md.