Release Google.Api.Gax version 3.6.0

Changes since 3.5.0:

- Support for REST-based gRPC adapter ("REGAPIC")
- Use self-signed JWTs in Google.Apis.Gax.Rest
- Extension methods on RpcException to access extra error information
- Updated dependencies

ReleaseVersion.xml

@@ -5,6 +5,6 @@
     - divergent versions, but for the moment this keeps things simpler.
     -->
   <PropertyGroup>
-    <Version>3.6.0-beta03</Version>
+    <Version>3.6.0</Version>
   </PropertyGroup>
 </Project>