Fix Postgres migration when schema is not public.

[adriennesox]

#6339

If the schema being used for HAPI in Postgres is not the public schema, the 7.4.0 migrations fails because the new DeletePrimaryKeyTask assumes the active schema is public when it looks up the primary key name.

This changes it to use current_schema() if available (i.e. the HAPI user has a search_path set) and 'public' otherwise.

Tested by running the 7.4.2 migrations with this change incorporated against a database where the old migrations were failing.

[tadgh]

Heya! Sorry it has taken me so long to get to this. Will review it today!

[tadgh]

Looks good to me! Looks like you're already attributed in the developers section, so I will go ahead and write a changelog for this after it merges 👍🏻

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.53%. Comparing base (406db33) to head (e03af57).
Report is 51 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6340      +/-   ##
============================================
- Coverage     83.54%   83.53%   -0.01%     
- Complexity    27432    27628     +196     
============================================
  Files          1707     1720      +13     
  Lines        106185   106943     +758     
  Branches      13397    13453      +56     
============================================
+ Hits          88710    89339     +629     
- Misses        11750    11827      +77     
- Partials       5725     5777      +52     

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