update taxi_utils not to use deprecated model save apis

tfx/examples/chicago_taxi_pipeline/taxi_utils.py

@@ -370,4 +370,4 @@ def run_fn(fn_args: fn_args_utils.FnArgs):
           model, tf_transform_output
       ),
   }
-  model.save(fn_args.serving_model_dir, save_format='tf', signatures=signatures)
+  tf.saved_model.save(model, fn_args.serving_model_dir, signatures=signatures)