unpin protobuf

[badGarnet]

• grpc requires protobuf>=5.26.1 https://github.com/grpc/grpc/blob/v1.68.0/requirements.txt#L4
• local inference was cited as a reason to pin protobuf but latest unstructured-inference do not pin the package and it is actually at 5.29.3 https://github.com/Unstructured-IO/unstructured-inference/blob/625255a810730100c09d5cfc6ff9618b26a6c368/requirements/base.txt#L92
• this PR unpins protobuf