[windows] Bump ffigen to 16.1.0

[Levi-Lesches]

Closes #2471 by manually bumping, re-running ffigen, then cleaning up the resulting Dart code.

The big change in ffigen here is that it better handles enums, by generating actual Dart enums instead of integers

[Levi-Lesches]

Well there you have it:

Because flutter_local_notifications_windows depends on ffigen >=15.0.0 which requires SDK version >=3.4.0 <4.0.0, version solving failed.

So to merge this, we'd have to bump the SDK to ^3.4.0 and Flutter ^3.22.0, which are the next versions of Dart and Flutter. It's not a terrible change, but this PR doesn't add any new functionality so it's also not a high priority.

Feel free to mark as a draft if you want to wait on this

[MaikuB]

Happy to bump to Flutter 3.22 as that then makes the minimum Android API 21. #2440 is dependent on Flutter 3.22 as well but held off so that would allow Flutter 3.22 to be at least the two stable releases behind

[Levi-Lesches]

Okay, confusingly, ffigen ^16.0.0 conflicts with Flutter 3.22 because of a conflict with the analyzer and meta packages. I've opened dart-lang/pub#4488 to discuss how this could be caught sooner because my computer, with the latest Flutter, doesn't show these issues. Not sure what to do except wait in this case.