[FrameworkBundle] Use static closure in front controller (8.1) (#1532)

Added missing static for framework-bundle 8.1.

Author: mbaschnitzi

symfony/framework-bundle/8.1/public/index.php

@@ -4,6 +4,6 @@
 
 require_once dirname(__DIR__).'/vendor/autoload_runtime.php';
 
-return function (array $context) {
+return static function (array $context) {
     return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
 };