Extract PGBouncer Failover Logic to Library

What does this MR do?

• Extracts the PGBouncer failover script logic into a library to allow unit testing and code sharing when support is added for multiple types of postgres instances
• Remove the last few vestiges of repmgr that were not removed when the Patroni changeover was finalized

Related issues

Related #6587 (closed)

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 #6708 (closed)