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

tests/phpunit/tests/robots.php

@@ -43,7 +43,7 @@ class Tests_Robots extends WP_UnitTestCase {
 	public function test_wp_robots_parses_directives_correctly() {
 		add_filter(
 			'wp_robots',
-			function( array $robots ) {
+			static function( array $robots ) {
 				// Directives that should have values must use strings.
 				$robots['directive-with-value']         = 'yes';
 				$robots['directive-with-numeric-value'] = '1';