Address pandas deprecation warnings in test suite

[kandersolar]

• [ ] Closes #xxxx
• I am familiar with the contributing guidelines
• I attest that all AI-generated material has been vetted for accuracy and is in compliance with the pvlib license
• Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
• [ ] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [ ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

A trivial PR to address these pandas warnings:

tests/test_pvsystem.py::test_combine_loss_factors
  /home/runner/work/pvlib-python/pvlib-python/tests/test_pvsystem.py:2425: Pandas4Warning: 'd' is deprecated and will be removed in a future version, please use 'D' instead.
    test_index = pd.date_range(start='1990/01/01T12:00', periods=365, freq='d')

tests/test_pvsystem.py::test_combine_loss_factors
  /home/runner/work/pvlib-python/pvlib-python/tests/test_pvsystem.py:2427: Pandas4Warning: 'd' is deprecated and will be removed in a future version, please use 'D' instead.
    loss_2 = pd.Series(.05, index=pd.date_range(start='1990/01/01T12:00',

https://github.com/pvlib/pvlib-python/actions/runs/27138420944/job/80096740936#step:9:90

[echedey-ls]

Did you check for other occurrences in the examples gallery? I have trauma with them being ignored in the test CI.

[echedey-ls]

Okay, now I remember. In #2730 - I thought it addressed the whole codebase, and #2664 must have addressed those there in the examples. LGTM :)