Pin protobuf version to 3.20.3 to fix an import error

[james47]

As title. This fixes the following:

ImportError Traceback (most recent call last) [<ipython-input-1-84f0d6543138>](https://localhost:8080/#) in <cell line: 6>() 4 import tensorflow_ranking as tfr 5 import tensorflow_text as tf_text ----> 6 from tensorflow_serving.apis import input_pb2 7 from google.protobuf import text_format [/usr/local/lib/python3.10/dist-packages/tensorflow_serving/apis/input_pb2.py](https://localhost:8080/#) in <module> 3 # source: tensorflow_serving/apis/input.proto 4 """Generated protocol buffer code.""" ----> 5 from google.protobuf.internal import builder as _builder 6 from google.protobuf import descriptor as _descriptor 7 from google.protobuf import descriptor_pool as _descriptor_pool ImportError: cannot import name 'builder' from 'google.protobuf.internal' (/usr/local/lib/python3.10/dist-packages/google/protobuf/internal/__init__.py)``` 

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