Clean-up files in repo

[ViktorHofer]

Minor clean-up in the repo:

• Use LangVersion=latest which most other repos use
• Use the standardized set of TFM properties NetCurrent, NetMinimum and NetFrameworkMinimum
• Set TreatWarningsAsErrors=true so that it applies to dotnet build as well
• Turn on CentralPackageTransitivePinningEnabled for NuGet Central Package Management
• Remove unused PackageVersion entries and update the xunit ones to the latest version used by Arcade
• Remove unused feeds in NuGet.config
• Remove unused files from the repo
• Remove unnecessary NoWarns
• Add a RollFoward=Major policy to dotnet-suggest so that the tool works not just with an installed net8.0 runtime.