Move constraints into `@platforms`

[UebelAndre]

Given that the constraints haven't changed in two years (f9b2e11), can these be moved into @platforms to avoid #463 ?

[keith]

I can't remember the specifics of the discussion but I don't think the platforms repo wants platform-specific add on constraints like this

[UebelAndre]

Can there maybe be a platforms_apple repository then which has the constraints?

[keith]

that would definitely work but i think it's likely that we'd prefer to solve the other issue in another way instead