protoc-rust-grpc dependency on protoc-rust is too loose

[mzabaluev]

With the dependency on protoc-rust being just "2" in the Cargo.toml of protoc-rust-grpc, a project state is allowed when protoc-rust is too old for the API that protoc-rust-grpc actually uses:

error[E0433]: failed to resolve: could not find `Codegen` in `protoc_rust`
   --> /home/mzabaluev/.cargo/registry/src/github.com-1ecc6299db9ec823/protoc-rust-grpc-0.8.0/src/lib.rs:103:26
    |
103 |             protoc_rust::Codegen::new()
    |                          ^^^^^^^ could not find `Codegen` in `protoc_rust`

The dependency should be upped to at least the minimum required version of protoc-rust that provides the Codegen API.