Resolve #3428282 "Humans for boost"

Closes #3428282

src/EventSubscriber/BoostResponseSubscriber.php

@@ -85,7 +85,7 @@ class BoostResponseSubscriber implements EventSubscriberInterface {
   /**
    * {@inheritdoc}
    */
-  public static function getSubscribedEvents() {
+  public static function getSubscribedEvents(): array {
     return [
       KernelEvents::REQUEST => [
         ['onRequest', 100],