message: grpc: officially deprecate Grpc::RpcChannelImpl and remove in 1.4.0.
  This will resolve when we remove the files, there are no longer any uses of this within the
  main Envoy repo, so this exists solely for consuming projects.
diff: @@ -17,3 +17,6 @@ The following features have been DEPRECATED and will be removed in the specified
  * The connectionId() and ssl() callbacks of StreamFilterCallbacks have been deprecated and
  replaced with a more general connection() callback, which, when not returning a nullptr, can be
  used to get the connection id and SSL connection from the returned Connection object pointer.
  + * The protobuf stub gRPC support via `Grpc::RpcChannelImpl` is now replaced with `Grpc::AsyncClientImpl`.
  + This no longer uses `protoc` generated stubs but instead utilizes C++ template generation of the
  + RPC stubs. `Grpc::AsyncClientImpl` supports streaming, in addition to the previous unary, RPCs.
  