improve project.json files

src/csharp/Grpc.Core.Tests/project.json

src/csharp/Grpc.Core/project.json

src/csharp/Grpc.Dotnet.sln

src/csharp/Grpc.Examples.MathClient/project.json

src/csharp/Grpc.Examples.MathServer/project.json

src/csharp/Grpc.Examples.Tests/project.json

src/csharp/Grpc.Examples/project.json

src/csharp/Grpc.HealthCheck.Tests/project.json

src/csharp/Grpc.HealthCheck/project.json

src/csharp/Grpc.IntegrationTesting.Client/project.json

src/csharp/Grpc.IntegrationTesting.QpsWorker/project.json

src/csharp/Grpc.IntegrationTesting.Server/project.json

...g.StressClient/Grpc.IntegrationTesting.StressClient.xproj

src/csharp/Grpc.IntegrationTesting.StressClient/project.json

src/csharp/Grpc.IntegrationTesting/project.json