add copyrights for AssemblyInfo.cs files

...ples/csharp/helloworld/Greeter/Properties/AssemblyInfo.cs

...sharp/helloworld/GreeterClient/Properties/AssemblyInfo.cs

...sharp/helloworld/GreeterServer/Properties/AssemblyInfo.cs

.../csharp/route_guide/RouteGuide/Properties/AssemblyInfo.cs

...p/route_guide/RouteGuideClient/Properties/AssemblyInfo.cs

...p/route_guide/RouteGuideServer/Properties/AssemblyInfo.cs

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

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

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

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

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

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

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

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

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

...Grpc.IntegrationTesting.Client/Properties/AssemblyInfo.cs

...c.IntegrationTesting.QpsWorker/Properties/AssemblyInfo.cs

...Grpc.IntegrationTesting.Server/Properties/AssemblyInfo.cs

...ntegrationTesting.StressClient/Properties/AssemblyInfo.cs

...csharp/Grpc.IntegrationTesting/Properties/AssemblyInfo.cs