Merge pull request #7054 from jtattermusch/csharp_update_examples_to_0_15

examples/csharp/helloworld/.nuget/packages.config

examples/csharp/helloworld/Greeter/Greeter.csproj

examples/csharp/helloworld/Greeter/Helloworld.cs

examples/csharp/helloworld/Greeter/HelloworldGrpc.cs

examples/csharp/helloworld/Greeter/packages.config

...ples/csharp/helloworld/GreeterClient/GreeterClient.csproj

examples/csharp/helloworld/GreeterClient/packages.config

...ples/csharp/helloworld/GreeterServer/GreeterServer.csproj

examples/csharp/helloworld/GreeterServer/packages.config

examples/csharp/helloworld/generate_protos.bat

examples/csharp/route_guide/.nuget/packages.config

examples/csharp/route_guide/RouteGuide/RouteGuide.cs

examples/csharp/route_guide/RouteGuide/RouteGuide.csproj

examples/csharp/route_guide/RouteGuide/RouteGuideGrpc.cs

examples/csharp/route_guide/RouteGuide/packages.config

...harp/route_guide/RouteGuideClient/RouteGuideClient.csproj

examples/csharp/route_guide/RouteGuideClient/packages.config

...harp/route_guide/RouteGuideServer/RouteGuideServer.csproj

examples/csharp/route_guide/RouteGuideServer/packages.config

examples/csharp/route_guide/generate_protos.bat