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.

git-svn-id: https://develop.svn.wordpress.org/trunk@55988 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-date-query.php

@@ -494,7 +494,7 @@ class WP_Date_Query {
 		);
 
 		// Attempt to detect a table prefix.
-		if ( false === strpos( $column, '.' ) ) {
+		if ( ! str_contains( $column, '.' ) ) {
 			/**
 			 * Filters the list of valid date query columns.
 			 *