Tutorial upgrade .net version

[vb2ae]

Please update the .net version of the tutorial to get rid of this warning

Warning: C:\Program Files\dotnet\sdk\8.0.403\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.EolTargetFrameworks.targets(32,5): warning NETSDK1138: The target framework 'netcoreapp3.1' is out of support and will not receive security updates in the future. Please refer to https://aka.ms/dotnet-core-support for more information about the support policy. [D:\a\Caliburn.Micro\Caliburn.Micro\samples\tutorals\WPF\Wpf.Tutorial\Caliburn.Micro.Tutorial.Wpf\Caliburn.Micro.Tutorial.Wpf.csproj]