Add migration guides for pytest-asyncio v0.21 and v0.23

[seifertm]

No description provided.

[seifertm]

@MarkusSintonen If you can spare the time, I'd appreciate a review on the migration guides for pytest-asyncio v0.24. If you have any comments on a general lack of documentation, we can address them in this PR, too.

https://pytest-asyncio--918.org.readthedocs.build/en/918/

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.30%. Comparing base (1bfc181) to head (1326c06).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #918   +/-   ##
=======================================
  Coverage   91.30%   91.30%           
=======================================
  Files           2        2           
  Lines         506      506           
  Branches       98       98           
=======================================
  Hits          462      462           
  Misses         26       26           
  Partials       18       18           

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