.NET 6 API-31 reference pack is not being updated

[jpobst]

Since we marked API-32 as stable, the .NET 6 API-31 reference pack is no longer being updated (newest is P12):
https://www.nuget.org/packages/Microsoft.Android.Ref.31

Because this is the default most users will use (net6.0-android), they are not seeing the fixes we have made:

• Deprecate AndroidClientHandler in .NET 6 #6541
• [Mono.Android] Fix setColor (int) / setColor (long) issue. #6617

While we should not update these after they are marked final, we should continue updating them during the preview program, as we will be making fixes to the Mono.Android API throughout the preview.

Workaround: Users can target API-32 (net6.0-android32.0) as that reference pack is being updated.