Use constant Routes::JSON_API_ROUTE_FLAG_KEY instead of hard-coded string

Closes #3172870

src/EventSubscriber/ResponseSubscriber.php

@@ -88,7 +88,7 @@ class ResponseSubscriber implements EventSubscriberInterface {
       return;
     }
     $route_defaults = $this->routeMatch->getRouteObject()->getDefaults();
-    if (Routes::isJsonApiRequest($route_defaults) || !empty($route_defaults['_is_jsonapi'])) {
+    if (Routes::isJsonApiRequest($route_defaults) || !empty($route_defaults[Routes::JSON_API_ROUTE_FLAG_KEY])) {
       $response = $event->getResponse();
       if ($response instanceof CacheableResponseInterface) {
         $response->getCacheableMetadata()->addCacheContexts(['url.query_args:jsonapi_include']);