Update prepull image version

update prepull image version

Author: jingxu97

test/k8s-integration/prepull.yaml

@@ -33,7 +33,7 @@ spec:
       # run ping in each container to keep it alive so that kubernetes does not
       # continually restart the containers while we're prepulling.
       containers:
-      - image: k8s.gcr.io/e2e-test-images/agnhost:2.21
+      - image: k8s.gcr.io/e2e-test-images/agnhost:2.26
         name: agnhost-212
         resources:
           requests: