Release Google.Api.CommonProtos version 2.12.0

Changes since 2.11.0:

- The package contains the original protos, in contents/protos.
  Build targets in the package set the `GoogleApiCommonProtos_ProtosPath`
  msbuild property, and if `IncludeGoogleApiCommonProtos` is set to
  the value `true` in the project, the `Protobuf_StandardImportsPath`
  property is updated to include it before the `Protobuf_BeforeCompile`
  target is executed. (This should make the package easier to work with
  from Grpc.Tools.)

Google.Api.CommonProtos/Google.Api.CommonProtos.csproj

@@ -3,7 +3,7 @@
   <Import Project="..\CommonProperties.xml" />
 
   <PropertyGroup>
-    <Version>2.11.0</Version>
+    <Version>2.12.0</Version>
     <TargetFrameworks>netstandard2.0;net461</TargetFrameworks>
   </PropertyGroup>