Code Modernization: Use str_starts_with() in WP_Duotone class methods.

This aims to make the code more readable and consistent, as the function is already used extensively in core files.

WordPress core includes a polyfill for `str_starts_with()` on PHP < 8.0 as of WordPress 5.9.

Follow-up to [55703], [58313].

Props Soean, mukesh27.
Fixes #64773.

git-svn-id: https://develop.svn.wordpress.org/trunk@61781 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

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

@@ -1193,7 +1193,7 @@ public static function restore_image_outer_container( $block_content ) {
 
 		$inner_classnames = explode( ' ', $tags->get_attribute( 'class' ) );
 		foreach ( $inner_classnames as $classname ) {
-			if ( 0 === strpos( $classname, 'wp-duotone' ) ) {
+			if ( str_starts_with( $classname, 'wp-duotone' ) ) {
 				$tags->remove_class( $classname );
 				$tags->seek( 'wrapper-div' );
 				$tags->add_class( $classname );