Remove grpc_utils/ backwards compatibility code.
We left headers, type aliases and code build script hacks to maintains backwards compatibility when we moved the symbols out of google::cloud::grpc_utils:: (and the files out of google/cloud/grpc_utils/).
We do not have specific plans to clean this up, for now this is just a placeholder to point the TODOs to.
This is an API breaking change, so we don't want to make this change until we're ready to make a breaking change.
Add #warning directives and start a timer to remove them.
I updated the files and announced they will be removed around 2023-01-01
Still waiting for the passage of time.