Drops PG15 support

[naisila]

DESCRIPTION: Drops PG15 support

Note to reviewer: You can review commit by commit, the commit message should be explanatory enough.

Fixes #8351

Sister PR https://github.com/citusdata/the-process/pull/178

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 88.73%. Comparing base (0e110ee) to head (4a5be6f). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8372      +/-   ##
==========================================
- Coverage   88.94%   88.73%   -0.21%     
==========================================
  Files         287      287              
  Lines       63153    63019     -134     
  Branches     7939     7891      -48     
==========================================
- Hits        56169    55922     -247     
- Misses       4671     4781     +110     
- Partials     2313     2316       +3     

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.