Default to PostgreSQL 12 in fresh installs

What does this MR do?

Sets PostgreSQL 12 as the default for new installations.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened

Closes #5509 (closed)

Testing Notes

1. install package fresh
2. confirm version 12 of PostgreSQL

   gitlab-psql
   select version();

3. Install current stable
4. confirm version 11 of PostgreSQL

   gitlab-psql
   select version();

5. Upgrade to this package, then run gitlab-cl reconfigure again

   gitlab-psql
   select version();

See comment thread for full output examples.