Where to get FormsViewGroup.dll for Android 6.0?

I created new Cross Platform App (Xamarin). I want target Android 6.0 Marshmallow.

On deploy I get error message:
The $(TargetFrameworkVersion) for FormsViewGroup.dll (v7.1) is greater than the $(TargetFrameworkVersion) for your project (v6.0). You need to increase the $(TargetFrameworkVersion) for your project.

But I do not want to target 7.1, I want to target 6.0, what can I do?

Thanks
Vaclav