[PSM Interop] Changing xds server to xDS control plane in error message (#34808)

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tools/run_tests/xds_k8s_test_driver/framework/test_app/client_app.py

@@ -330,7 +330,7 @@ class XdsTestClient(framework.rpc.grpc.GrpcApp):
 
         raise self.ChannelNotActive(
             f"[{self.hostname}] Client has no"
-            f" active channel with xds server {xds_server_uri}",
+            f" active channel with xDS control plane {xds_server_uri}",
             src=self.hostname,
             dst=xds_server_uri,
         )