introduce Version.cs as single source of truth of gRPC C# version

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

src/csharp/Grpc.Auth/Properties/AssemblyInfo.cs

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

src/csharp/Grpc.Core.Tests/Properties/AssemblyInfo.cs

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

src/csharp/Grpc.Core/Properties/AssemblyInfo.cs

src/csharp/Grpc.Core/Version.cs

.../Grpc.Examples.MathClient/Grpc.Examples.MathClient.csproj

...sharp/Grpc.Examples.MathClient/Properties/AssemblyInfo.cs

.../Grpc.Examples.MathServer/Grpc.Examples.MathServer.csproj

...sharp/Grpc.Examples.MathServer/Properties/AssemblyInfo.cs

src/csharp/Grpc.Examples.Tests/Grpc.Examples.Tests.csproj

src/csharp/Grpc.Examples.Tests/Properties/AssemblyInfo.cs

src/csharp/Grpc.Examples/Grpc.Examples.csproj

src/csharp/Grpc.Examples/Properties/AssemblyInfo.cs

...harp/Grpc.HealthCheck.Tests/Grpc.HealthCheck.Tests.csproj

src/csharp/Grpc.HealthCheck.Tests/Properties/AssemblyInfo.cs

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

src/csharp/Grpc.HealthCheck/Properties/AssemblyInfo.cs

...ationTesting.Client/Grpc.IntegrationTesting.Client.csproj