add note when we can remove this BC hack again

Author: dbu

tests/Resources/app/AppKernel.php

@@ -66,6 +66,7 @@ public function loadRoutes(LoaderInterface $loader): RouteCollection
         $collection->add('/', new Route('/', ['_controller' => 'kernel::indexAction']));
 
         $routes = new RoutingConfigurator($collection, $kernelLoader, $file, $file);
+        // TODO hardcode xml when we support only Symfony 7.3 or higher.
         $extension = self::MAJOR_VERSION >= 7 ? 'php' : 'xml';
         $routes->import('@WebProfilerBundle/Resources/config/routing/wdt.'.$extension)->prefix('_wdt');
         $routes->import('@WebProfilerBundle/Resources/config/routing/profiler.'.$extension)->prefix('_profiler');