Investigate migration to Godot 4.0

[WildWeazel]

Godot 4.0 Beta 1 was just released(!), so I'm going to go ahead and put this on our plate. Given where we are in development and the amount of time we've spent on build issues, I'd assume it's probably worth migrating if only for full .NET 6 support. No ETA on a stable release, but we can at least scope out the effort. The next big step: Godot 4.0 reaches Beta https://godotengine.org/article/dev-snapshot-godot-4-0-beta-1#csharp