Apply Obsolete attribute to deprecated services and methods in C#
generated code
Fix for https://github.com/grpc/grpc/issues/28597
- Deprecated support for enums and enum values is already fixed by
https://github.com/protocolbuffers/protobuf/pull/10520 but this is not
yet released. It is fixed in Protocol Buffers v22.0-rc1 but the gRPC
repo currently has 21.12 as the protocol buffers submodule.
- Deprecated support for messages and fields already exists in the
protocol buffers compiler.
The fix in this PR adds `Obsolete` attribute to classes and methods for
deprecated services and methods within services. e.g.
```
service Greeter {
option deprecated=true; // service level deprecated
// Sends a greeting
rpc SayHello (HelloRequest) returns (HelloReply) {
option deprecated=true; // method level deprecated
}
}
```
I couldn't find any protocol buffers plugin tests to update. Tested
locally.
tony