Remove vestiges of PG11 Geo Support

What does this MR do?

Removes the last pieces within the Geo codebase that supported the use of repmgr for Geo high availability.

Related issues

1. Add prevention of upgrade if not using at least Postgres version 12
2. Remove Geo's ability to install with repmgr
3. Remove PostgreSQL 12 check in replicate-geo-database
4. Decouple rempgr from Postgres Role
5. Remove repmgr from Omnibus
6. Remove Postgres 11 from Omnibus
7. Update Postgres Documentation reflecting removal

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 #6175 (closed)