{CI} Add suppression for dummy DOCKER_REGISTRY_SERVER_PASSWORD in unit tests (#33279)

Author: wangzelin007

scripts/ci/credscan/CredScanSuppressions.json

@@ -695,6 +695,10 @@
     {
       "placeholder": "client_id_456",
       "_justification": "[ARO] Dummy client_id value in test_validators.py unit tests"
+    },
+    {
+      "placeholder": "mypass",
+      "_justification": "[AppService] Dummy DOCKER_REGISTRY_SERVER_PASSWORD value in test_compose_convert.py unit tests"
     }
   ]
 }