Raise minimum haxe version from 4.2.5 to 4.3.X

[Geokureli]

Continuing off of #2774. It's still far too early to switch to 4.3 as a minimum target, it hasn't even been a month and adoption for 4.3 will be notably slower than 4.2 since there are so many deprecation warnings and odd errors that have turned up, notably #2780

Can use self in abstracts that reference themself, example: https://github.com/HaxeFlixel/flixel/pull/2802

• Nice, but has no actual benefit to users

[Geokureli]

there's a bug in Haxe 4.2.5, 4.3.1 and nightly, preventing us from making FlxBaseTilemap and FlxTypedTilemap an abstract class. I do want to make that change but it will have to wait