Bundled Themes: Standardize the Required PHP and Tested Up To headers.

This change makes several changes to ensure consistency and accuracy for default theme headers:

- Removes `WordPress` from `Requires at least` headers.
- Ensures the `Requires at least` and `Requires PHP` headers are present in every default theme’s `style.css` file.
- Ensures `Tested up to` is present in every `readme.txt` file.
- Removes any headers not processed in both the `style.css` and `readme.txt` files for each theme.

Props Otto42, afragen.
Fixes #48517.
Built from https://develop.svn.wordpress.org/trunk@46676


git-svn-id: https://core.svn.wordpress.org/trunk@46476 1a063a9b-81f0-0310-95a4-ce76da25c4cd