Code Modernization: Use str_contains() in a few more places.

`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.

git-svn-id: https://develop.svn.wordpress.org/trunk@56031 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-editor.php

@@ -571,7 +571,7 @@ final class _WP_Editors {
 					if ( ! empty( $editor_styles ) ) {
 						// Force urlencoding of commas.
 						foreach ( $editor_styles as $key => $url ) {
-							if ( strpos( $url, ',' ) !== false ) {
+							if ( str_contains( $url, ',' ) ) {
 								$editor_styles[ $key ] = str_replace( ',', '%2C', $url );
 							}
 						}