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/canonical.php

@@ -247,7 +247,7 @@ class Tests_Canonical extends WP_Canonical_UnitTestCase {
 		// Test short-circuit filter.
 		add_filter(
 			'pre_redirect_guess_404_permalink',
-			function() {
+			static function() {
 				return 'wp';
 			}
 		);