drop support for Django versions below 4.2

[dulmandakh]

Description of the Change

Django versions below 4.2 are no longer supported, so this PR removes mentions and CI for these versions.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.32%. Comparing base (51d9798) to head (75c28f8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1455      +/-   ##
==========================================
- Coverage   97.34%   97.32%   -0.03%     
==========================================
  Files          32       32              
  Lines        2112     2128      +16     
==========================================
+ Hits         2056     2071      +15     
- Misses         56       57       +1     

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[dulmandakh]

@n2ygk please review and merge. I have no idea why codecov is failing. Let's make CI lean and clean.

[n2ygk]

Yeah codecov keeps misreporting that tiny delta but it's unclear why. Luckily it done's block accepting the PR.

[n2ygk]

Nice work! Thanks for simplifying the workfliws.

One question regarding python 3.8, 3.9 needs to be resolved.