Coding Standards: Use static closures when not using $this.

When a closure does not use `$this`, it can be made `static` for improved performance.

Static closures are supported in PHP since PHP 5.4. ​

Props jrf, hellofromTonya, swissspidy, SergeyBiryukov.
See #53359.

git-svn-id: https://develop.svn.wordpress.org/trunk@51657 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/update-core.php

@@ -1658,7 +1658,7 @@ function _upgrade_422_find_genericons_files_in_folder( $directory ) {
 	$dirs = glob( $directory . '*', GLOB_ONLYDIR );
 	$dirs = array_filter(
 		$dirs,
-		function( $dir ) {
+		static function( $dir ) {
 			// Skip any node_modules directories.
 			return false === strpos( $dir, 'node_modules' );
 		}