chore!: Upgrade `protobuf`

[liuverta]

Impact and Context

This matches the protobuf version range required by the latest version of grpcio-tools, which we are now using for protobuf generation (https://github.com/VertaAI/protos-all/pull/528).

Risks and Area of Effect

• [x] Is this a breaking change?

I'm considering it a breaking change because the client will (probably) no longer work in Python environments using protobuf<4.0. Verta engineers using pip install -e will need to re-install the client.

But this should be a safe change to make today. Support for protobuf<4.0 ended July 2023, and protobuf==4.21.6 was released just over a year ago. Honestly it's a miracle (or just Python things™) that anyone's environment is able to run the client right now.

Testing

• [ ] Unit test
• [ ] Deployed to dev env
• [ ] Other (explain)

Reverting

• [ ] Contains Migration - Do Not Revert

Revert this PR.