ci-cd: Investigate using net6 to build netstandard2.0 Authgear.Xamarin

[roxk]

Since .NET6, multi-target libraries support is built-in. No need to use Extras build type. However, we need to see if using .NET 6 to build would cause the library to only be consumable by .NET 6. Theoretically, it shouldn't, due to .net standard, but it's not sure whether xamarin.ios and xamarin.android support working downlevel

• Try to convert authgear.xamarin to use single SDK-style project