nuget package icon, use recommended license tag

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Core.Api/Grpc.Core.Api.csproj

...csharp/Grpc.Core.NativeDebug/Grpc.Core.NativeDebug.csproj

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.csproj

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.HealthCheck/Grpc.HealthCheck.csproj

src/csharp/Grpc.Reflection/Grpc.Reflection.csproj

src/csharp/Grpc.Tools/Grpc.Tools.csproj

src/csharp/Grpc/Grpc.csproj