Deprecate Python 3.9 support

[SkafteNicki]

What does this PR do?

Part of #21304
Python 3.9 reached status end-of-life as of oct. 25 so lets remove support for it.
Instead 3.13 + 3.14 support is being added in other PRs.

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📚 Documentation preview 📚: https://pytorch-lightning--21398.org.readthedocs.build/en/21398/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (79ffe50) to head (b90bf96).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21398   +/-   ##
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  Coverage      87%      87%           
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  Files         269      269           
  Lines       23804    23804           
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  Hits        20626    20626           
  Misses       3178     3178