PG18: drop constraints ONLY and NOT VALID fks on partitioned tables

[colm-mchugh]

This commit verifies that PG18 behavior relating to partitioned tables is propagated and behaves consistently on Citus distributed tables:

• Allow dropping of constraints ONLY on partitioned tables
• Allow NOT VALID foreign key constraints on partitioned tables

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 88.94%. Comparing base (529b303) to head (4ea5a7d). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8374   +/-   ##
=======================================
  Coverage   88.93%   88.94%           
=======================================
  Files         287      287           
  Lines       63154    63153    -1     
  Branches     7938     7938           
=======================================
+ Hits        56169    56171    +2     
+ Misses       4673     4670    -3     
  Partials     2312     2312           

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