Incompatible with projects using protobuf 2.28

In Cargo dependency versions are deduplicated across the entire workspace (per each major version). The hardcoded dependency protobuf in tensorflow forces all users of tensorflow to use exactly this version of protobuf 2.27.1. It causes a fatal error when resolving dependencies if any dependency anywhere wants protobuf 2.28.

Unfortunately, this version requirement is very public and global. In Cargo the exact = version requirements are quite disruptive. Please don't hardcode the version. If you can't use 2.28, consider forking the crate and depending on the fork, which will not conflict with other uses of the protobuf crate.

https://github.com/tensorflow/rust/blob/62a3019cfa0720c9744b4bd5f97db953bf4066cf/Cargo.toml#L31-L32