Bump to xamarin/java.interop/master@a8f68e56

Changes: dotnet/java-interop@3894cd7...a8f68e5

* Use `const` instead of `static readonly`
* [generator] Disable [SupportedOSPlatform] until .NET 5/6.
* [generator] Wrap SupportedOSPlatformAttribute in NET
* [build] use $(RollForward)=Major for console apps
* [Java.Interop] Conditionally call ManagedPeer.Init()
* [generator] Add [SupportedOSPlatform] in assemblies using ApiSince
* [global.json] Update Microsoft.Build.NoTargets to 2.0.1.

Author: jonathanpeppers