Update readthedocs config

[kandersolar]

• [ ] Closes #xxxx
• [ ] I am familiar with the contributing guidelines
• [ ] 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.
• [x] Pull request is nearly complete and ready for detailed review.
• [x] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

RTD doesn't like this project's configuration file. See the warnings on this build: https://readthedocs.org/projects/solarfactors/builds/21172217/

Your builds will stop working soon! "build.image" config key is deprecated and it will be removed soon. Read our blog post to know how to migrate to new key "build.os" and ensure your project continues building successfully.

Your builds will stop working soon! Configuration files will soon be required by projects, and will no longer be optional. Read our blog post to create one and ensure your project continues building successfully.

This PR updates the configuration file to the current recommended state.

[echedey-ls]

Any reason this got delayed? Porting pvlib org packages to share a common infrastructure seems like a priority in the long term.

[kandersolar]

I probably just never came back to merge it after waiting for the CI build to complete. Thanks for the reminder @echedey-ls :)