Clean up Mono support and replace it with robust .NET 8 support

[Bartleby2718]

Filed per #111 (comment)

[Bartleby2718]

@madelson

1. Given https://github.com/madelson/MedallionShell/blob/release-1.7/MedallionShell/WindowsCommandLineSyntax.cs#L15-L16, it seems like we can drop https://github.com/madelson/MedallionShell/blob/release-1.7/MedallionShell/MonoUnixCommandLineSyntax.cs entirely. Does that sound right?
2. https://github.com/madelson/MedallionShell/blob/release-1.7/MedallionShell/PlatformCompatibilityHelper.cs#L51-L76 mentions Mono-specific issues. Can this now be ignored?