Drop Windows 9x support

Drop Windows 9x support

- Windows 98 was using `%windir%\Application Data` without defining `%APPDATA%`
- Writing user config in `C:\netradiant` looked fine on single user OS driving FAT file system
- We can't build for Windows platforms that does not support `%APPDATA%`
- Windows XP Schrödinger edition is able to define `%APPDATA%` while being dead at the same time
- Even ReactOS 3.11 for Workgroups defines `%APPDATA%`
- Wine defines `%APPDATA%` like a boss
- Twenty first century is now a thing
- NetRadiant will never run on OpenGEM/FreeDOS (and that's a shame)