.NET for Android error/warning XA4313
The specified Framework assembly has been deprecated. You need an upgrade to use the specified NuGet Package.
For example, to upgrade OpenTK-1.0
remove
<Reference Include="OpenTK-1.0" />
from your csproj then add
<PackageReference Include="Xamarin.Legacy.OpenTK" Version="1.0.0" />
to your project to upgrade.