💥 Reduce compatibility to Django 4.2

[jambonrose]

In anticipation of django-improved-user v3, which will be compatible with Django 4.2, 5.0, and 5.1, first:

• reduce the test suite to just Django 4.2
  (removing Django 2.2, 3.0, 3.1, 3.2, 4.0, 4.1)
• remove compatibility with Python 3.6 and 3.7
• upgrade Ubuntu test suite runners 20.04 → 22.04
• remove deprecated Django features

Note that tests no longer run on the integration example project. This is largely because the project is structured to use django-registration v2, which is not compatible with Django above v2.2. An improved test-suite for the integration material is forthcoming.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100%. Comparing base (2ff2ef4) to head (233b885).
Report is 1 commits behind head on development.

Additional details and impacted files

@@            Coverage Diff             @@
##           development   #303   +/-   ##
==========================================
  Coverage          100%   100%           
==========================================
  Files                9      9           
  Lines              174    173    -1     
  Branches            32      9   -23     
==========================================
- Hits               174    173    -1     

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