Upgrade project to 8.0 or later

[unitycoder]

this would allow using better Select Folder dialog (and maybe other speed improvements) https://devblogs.microsoft.com/dotnet/wpf-file-dialog-improvements-in-dotnet-8/

Previously i've tested upgrading, but many things broke down..

• [ ] test upgrade to 8.0 or later (currently uses 4.8)
• [ ] check that all features work
• [ ] can it build single exe builds?
• [ ] update github actions
• [ ] update installer builds

Branch: (if anyone wants to help solving those errors there, oO) https://github.com/unitycoder/UnityLauncherPro/tree/upgrade-to-NET8