chore(deps): drop support for Django versions not supported upstream

[nijel]

In case somebody wants to use that, older versions are available, but we will not keep maintaining the compatibility in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.73%. Comparing base (fb93814) to head (ebad193).
Report is 129 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #674      +/-   ##
==========================================
+ Coverage   91.72%   91.73%   +0.01%     
==========================================
  Files          39       40       +1     
  Lines        1172     1186      +14     
  Branches      144       66      -78     
==========================================
+ Hits         1075     1088      +13     
- Misses         72       73       +1     
  Partials       25       25              

Flag	Coverage Δ
unittests	91.73% <ø> (+0.01%)	⬆️

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