Twenty Twenty-One: Revert usage of str_contains() in theme files.

The theme supports WordPress 5.3 or later, while the polyfill for `str_contains()` only exists in WordPress 5.9 or later.

Follow-up to [55988], [57275], [57276].

Props poena.
Fixes #60241.

git-svn-id: https://develop.svn.wordpress.org/trunk@57277 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-content/themes/twentytwentyone/inc/template-functions.php

@@ -445,7 +445,7 @@ function twenty_twenty_one_get_attachment_image_attributes( $attr, $attachment,
 		return $attr;
 	}
 
-	if ( isset( $attr['class'] ) && str_contains( $attr['class'], 'custom-logo' ) ) {
+	if ( isset( $attr['class'] ) && false !== strpos( $attr['class'], 'custom-logo' ) ) {
 		return $attr;
 	}