chore: drop support for Django 4.2, 5.0 and Python 3.9

[nijel]

• Django 5.0 is no longer supported upstream.
• Django 4.2 is still LTS, but it is no longer possible to keep compatibility with upcoming Django 6 and 4.2 (see Add non-empty constraint for UserSocialAuth uid #557).
• Python 3.9 was supported only with Django 4.2.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.32%. Comparing base (fb93814) to head (5e1feb2).
Report is 158 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
+ Coverage   91.72%   92.32%   +0.60%     
==========================================
  Files          39       40       +1     
  Lines        1172     1173       +1     
  Branches      144       66      -78     
==========================================
+ Hits         1075     1083       +8     
+ Misses         72       65       -7     
  Partials       25       25              

Flag	Coverage Δ
unittests	92.32% <100.00%> (+0.60%)	⬆️

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