Replacing gRPC.Core with gRPC.Dotnet

[Lexcess]

Given the update here: https://grpc.io/blog/grpc-csharp-future/ with gRPC.Core being phased out. Is there any chance the c# sample could be updated to the new gRPC-Dotnet? I believe it is mostly just a case of swapping out packages and namespaces (but could be wrong!).

This repo is much appreciated, it'd be great to have an update. If it isn't possible, I'll see if I can get a PR together at some point, but I can't promise anything at this point.