Fix for lightning tests

[mrwyattii]

Getting the following error with recent lightning tests:

TypeError: Descriptors cannot not 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If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc 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).

See Lightning-AI/pytorch-lightning#13159 and protocolbuffers/protobuf#10051

The protobuf release causing the issue was also yanked: https://pypi.org/project/protobuf/4.21.0/

This PR pins the version of protobuf to <4.21.0 for now

[jeffra]

/cc @SeanNaren