Update `protobuf` version

[paolodelia99]

Hello,

I noticed that when installing tff on Google Colab, I encounter the following error when trying to import it:

TypeError: Descriptors cannot be created directly.  
If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc >= 3.19.0.  
If you cannot immediately regenerate your protos, some other possible workarounds are:  
 1. Downgrade the protobuf package to 3.20.x or lower.  
 2. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python (but this will use pure-Python parsing and will be much slower).  

To work around this, I have to manually install protobuf>=3.20.3 and restart the session, which disrupts the workflow.

Would it be possible to update the protobuf dependency in the package to avoid this issue? This would improve the out-of-the-box experience, especially for users running the library in cloud-based environments like Colab.

Thanks!