Merge pull request #4162 from jtattermusch/bump_csharp_version

bump C# version to 0.12.0

src/csharp/Grpc.Core/VersionInfo.cs

@@ -41,6 +41,6 @@ namespace Grpc.Core
         /// <summary>
         /// Current version of gRPC C#
         /// </summary>
-        public const string CurrentVersion = "0.7.1";
+        public const string CurrentVersion = "0.12.0";
     }
 }

src/csharp/build_packages.bat

 @rem Builds gRPC NuGet packages
 
 @rem Current package versions
-set VERSION=0.7.1
-set CORE_VERSION=0.11.1
+set VERSION=0.12.0
+set CORE_VERSION=0.12.0
 set PROTOBUF_VERSION=3.0.0-alpha4
 
 @rem Packages that depend on prerelease packages (like Google.Protobuf) need to have prerelease suffix as well.