ci: add Python 3.14 to test matrix

[matsumotosan]

What does this PR do?

Fixes #21304

• Python 3.9 support already deprecated
• Test Python 3.14 in CI

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📚 Documentation preview 📚: https://pytorch-lightning--21702.org.readthedocs.build/en/21702/

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (78bf021) to head (ae03cdd).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21702   +/-   ##
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  Coverage      87%      87%           
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  Files         270      270           
  Lines       23973    23973           
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  Hits        20748    20748           
  Misses       3225     3225