Add storage migration image to Knative Serving 0.15.

Author: markusthoemmes

ci-operator/config/openshift/knative-serving/openshift-knative-serving-release-v0.15.2__4.5.yaml

@@ -68,6 +68,14 @@ images:
       - destination_dir: .
         source_path: /go/bin/autoscaler
   to: knative-serving-autoscaler
+- dockerfile_path: openshift/ci-operator/knative-images/migrate/Dockerfile
+  from: base
+  inputs:
+    bin:
+      paths:
+      - destination_dir: .
+        source_path: /go/bin/migrate
+  to: knative-serving-storage-version-migration
 - dockerfile_path: openshift/ci-operator/knative-test-images/pizzaplanetv2/Dockerfile
   from: base
   inputs: