CLI: resolve jsonargparse deprecation warning

[adamjstewart]

What does this PR do?

The latest jsonargparse release deprecates set_config_read_mode in favor of set_parsing_settings. This results in the following warning message during testing:

../../../../../opt/hostedtoolcache/Python/3.13.3/x64/lib/python3.13/site-packages/lightning/pytorch/cli.py:52
  /opt/hostedtoolcache/Python/3.13.3/x64/lib/python3.13/site-packages/lightning/pytorch/cli.py:52: 
      By default only one JsonargparseDeprecationWarning per type is shown. To see all warnings set environment
      variable JSONARGPARSE_DEPRECATION_WARNINGS=all and to disable the warnings set
      JSONARGPARSE_DEPRECATION_WARNINGS=off.

../../../../../opt/hostedtoolcache/Python/3.13.3/x64/lib/python3.13/site-packages/lightning/pytorch/cli.py:52
  /opt/hostedtoolcache/Python/3.13.3/x64/lib/python3.13/site-packages/lightning/pytorch/cli.py:52: 
      set_config_read_mode was deprecated in v4.39.0 and will be removed in v5.0.0. Optional config read modes
      should now be set using function set_parsing_settings.

This PR defaults to the new function when available and falls back to the old function for older versions. This makes the code more futureproof and compatible with jsonargparse v5 once it is released. Let me know if you would rather replace the try-except with a version check.

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📚 Documentation preview 📚: https://pytorch-lightning--20802.org.readthedocs.build/en/20802/