Changeset 55988

Code Modernization: Replace usage of strpos() with str_contains().

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.