e2e-test: update-manifest-app3 (#3497)

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Author: argocd-commenter-e2e-setup[bot]

app3/cronjob/echoserver.yaml

@@ -14,7 +14,7 @@ spec:
           restartPolicy: Never
           containers:
             # https://hub.docker.com/_/busybox/
-            - image: busybox:1.28
+            - image: busybox:1.30
               name: echoserver
               command:
                 - /bin/sh