Fix getSubscribedEvents() compatibility

The current module version is not compatibile with latest D10.1.x due to a wrong function declaration which throws the following error:

Declaration of Drupal\menu_perms_per_menu\Routing\MenuPermsPerMenuRouteSubscriber::getSubscribedEvents() must be compatible with Drupal\Core\Routing\RouteSubscriberBase::getSubscribedEvents(): array

The attached patch fix the error.