Django to 6.0 support

[jrobichaud]

Closes #957

• drop django 5.0 and 5.1
• add django 6 task example
• actually add the support
• add documentation
• test with a real task backend

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (16b2fd5) to head (ec22804).

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #986    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           51        56     +5     
  Lines         2228      2638   +410     
  Branches        35        40     +5     
==========================================
+ Hits          2228      2638   +410     

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