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/includes/plural-form-function.php

@@ -7,7 +7,7 @@
  * @param string $expression
  */
 function tests_make_plural_form_function( $nplurals, $expression ) {
-	$closure = function ( $n ) use ( $nplurals, $expression ) {
+	$closure = static function ( $n ) use ( $nplurals, $expression ) {
 		$expression = str_replace( 'n', $n, $expression );
 
 		// phpcs:ignore Squiz.PHP.Eval -- This is test code, not production.