`str_contains()` was introduced in PHP 8.0 to perform a case-sensitive check indicating if the string to search in (haystack) contains the given substring (needle).
WordPress core includes a polyfill for `str_contains()` on PHP < 8.0 as of WordPress 5.9.
This commit replaces `false !== strpos( ... )` with `str_contains()` in core files, making the code more readable and consistent, as well as better aligned with modern development practices.
Follow-up to [52039], [52040], [52326], [55703], [55710], [55987].
Props Soean, spacedmonkey, costdev, dingo_d, azaozz, mikeschroder, flixos90, peterwilsoncc, SergeyBiryukov.
Fixes#58206.
Built from https://develop.svn.wordpress.org/trunk@55988
git-svn-id: http://core.svn.wordpress.org/trunk@55500 1a063a9b-81f0-0310-95a4-ce76da25c4cd
1ce5dc7444