[player] fix property name

Player/Serializer/ScenarioSetSerializer.php

@@ -156,7 +156,7 @@ private function cleanStepForJsonView(array &$step): void
             'errors' => true,
             'deprecations' => true,
             'initiator_uuid' => true,
-            'created_at' => true,
+            'started_at' => true,
             'finished_at' => true,
         ]);