Default PostgreSQL from 12.10 to 13.6

What does this Merge Request do and why?

Defaults PostgreSQL from 12.10 to 13.6.

Blocked by !2617 (merged)

Blocked by !2648 (merged)

Closes #1505 (closed)

cf. !2549 (merged), !2531 (merged)

Merge Request checklist

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