Update `protobuf` version

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If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc 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!

Start by locating the package dependency metadata and reproduce the reported import failure in a clean environment resembling Google Colab. Update the protobuf dependency so installation no longer requires a manual upgrade, then verify that installing the package and importing it succeeds without the descriptor error.

Written by the indexing model from the issue text.