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