Editor: Use static closures to avoid memory leaks.

Backports Gutenberg changes from https://github.com/WordPress/gutenberg/pull/50723.

Amends [55822].

Fixes #58323.
Props westonruter, spacedmonkey, flixos90. 


git-svn-id: https://develop.svn.wordpress.org/trunk@56038 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/media.php

@@ -4217,7 +4217,7 @@ EOF;
 		$result = null;
 		add_filter(
 			'the_content',
-			function( $content ) use ( &$result, $context ) {
+			static function( $content ) use ( &$result, $context ) {
 				$result = wp_get_loading_attr_default( $context );
 				return $content;
 			}