[AOT] Clean up gRPC template

[JamesNK]

• [x] Remove the preview version of Google.Protobuf in csproj. Once a Grpc.AspNetCore version is available which brings in an AOT-compatible Google.Protobuf then it's no longer required.
• [x] Ensure the -aot flag only sets required properties in csproj. Are trim mode and IIS asset publishing values required?

[ghost]

Thanks for contacting us.

We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s). If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.