move protobuf version to floating

[chayim]

attempting to update protobuf to be more variable. At the very least this helps solve issues where a newer version of protobuf is needed (due to ortools and other python versions).

[FrancoCV]

Hi! 👋 I'm interested in this feature and noticed the PR is still pending, likely due to a test failure in the minimal TF/Keras configuration. I'd love to see it merged — and if helpful, I'm happy to contribute a fix or open a follow-up PR based on this branch.

In our company, we’re currently working on an application that requires support for protobuf >= 4.25.8. However, tf2onnx 1.16.1 depends on protobuf >=3.20,<4.0, which causes version resolution to fail:

Because tf2onnx (1.16.1) depends on protobuf (>=3.20,<4.0) and no versions of tf2onnx match >1.16.1,<2.0.0, tf2onnx (>=1.16.1,<2.0.0) requires protobuf (>=3.20,<4.0).
So, because our app depends on both tf2onnx (^1.16.1) and protobuf (>=4.25.8), version solving failed.

This PR seems to solve that compatibility issue, so it would be very helpful for us to move it forward.

Thanks a lot for your work on this!