Twenty Nineteen: Standardize the Required PHP and Tested Up To headers.

* Remove `WordPress` from `Requires at least` headers.
* Ensure the `Requires at least` and `Requires PHP` headers are present in the `style.css` file.

Follow-up to [46676], which updated `style-rtl.css`, but not `style.scss` or `style.css`.

See #48517.
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