Deprecate `sky_diffuse_passias` and amend `plot_passias_diffuse_shading.py` gallery example

[kdebrab]

• [x] Closes #2584
• [x] I am familiar with the contributing guidelines
• [ ] ~~Tests added~~
• [ ] ~~Updates entries in docs/sphinx/source/reference for API changes.~~
• [x] 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.~~
• [x] 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.