`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 [55988], [56021].
See #58206.
Built from https://develop.svn.wordpress.org/trunk@56031
git-svn-id: http://core.svn.wordpress.org/trunk@55543 1a063a9b-81f0-0310-95a4-ce76da25c4cd