This affects:
* `validate_plugin_requirements()`
* `validate_theme_requirements()`
Historically, the `Requires PHP` header was introduced in #meta2952 for the Plugin Directory first, so at the time it made sense to have it defined in the same place as `Requires at least`, which only existed in `readme.txt`.
Since parsing of PHP and WordPress requirements was later added to WordPress core, the core should retrieve all the necessary data from the main plugin or theme file and not from `readme.txt`, which only contains the data meant for the Plugin or Theme Directory.
The recommended place for `Requires PHP` and `Requires at least` headers is as follows:
* The plugin's main PHP file
* The theme's `style.css` file
The place for the `Tested up to` header remains in `readme.txt` for the time being, as it's not used by WordPress core.
Follow-up to [44978], [45546], [47573], [47574], [meta5841], [meta9050].
Props afragen, Otto42, joyously, williampatton, audrasjb.
Fixes#48520. See #48515, #meta2952, #meta4514, #meta4621.
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