add missing Object-feed-created index

fixes https://console.cloud.google.com/errors/detail/CO-fpZfMq_6ktgE;time=P30D?project=bridgy-federated

index.yaml

@@ -82,6 +82,11 @@ indexes:
   - name: updated
     direction: desc
 
+- kind: Object
+  properties:
+  - name: feed
+  - name: created
+
 - kind: Object
   properties:
   - name: feed