nnyeah is a tool that can convert legacy Xamarin platform assemblies to run under .NET 6 or later. The repository for it is here: https://github.com/xamarin/xamarin-macios/tree/main/tools/nnyeah
nnyeah takes 3 inputs:
1. an assembly compiled against a Xamarin platform assembly
2. a Xamarin platform assembly (Xamarin.iOS.dll or Xamarin.macOS.dll)
3. a Microsoft platform assembly (Microsoft.iOS.dll or Microsoft.macOS.dll)
and will generate an output assembly which should act as it were compiled against the Microsoft platform assembly.