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-includes/rest-api/endpoints/class-wp-rest-controller.php

@@ -597,7 +597,7 @@ abstract class WP_REST_Controller {
 		// Return the list of all requested fields which appear in the schema.
 		return array_reduce(
 			$requested_fields,
-			function( $response_fields, $field ) use ( $fields ) {
+			static function( $response_fields, $field ) use ( $fields ) {
 				if ( in_array( $field, $fields, true ) ) {
 					$response_fields[] = $field;
 					return $response_fields;