Add missing return types to interfaces

Author: derrabus

Tests/Service/ServiceSubscriberTraitTest.php

@@ -76,10 +76,7 @@ public function aParentService(): Service1
     {
     }
 
-    /**
-     * @return ContainerInterface|null
-     */
-    public function setContainer(ContainerInterface $container)
+    public function setContainer(ContainerInterface $container): ?ContainerInterface
     {
         return $container;
     }