Consider not suggesting the `mach` crate on the deprecation note

[JohnTitor]

#1364 deprecated mach-related items and added a note suggesting the mach crate instead. But that crate hasn't been maintained since 2019 (related issue: fitzgen/mach#63), we should consider not suggesting it and how we treat mach-related items (for now, I've accepted #2283).
EDIT: I'm contacting the repo owner to see if I can help maintain.

[JohnTitor]

The current problem is that it's unclear what version we're referring to. So, are the mach-related items updated often? If so, we could add a cfg and separate declaration into "stable" ones and "unstable" ones (but it makes the code verbose).

[JohnTitor]

I haven't received any response from the repo owner for a week and I'm thinking of this.
So, the initial thought should've been that the values on mach.h were often updated/deprecated and versioning here didn't fit it. If this situation still continues, we have some ways to account for it:

1. Add a new feature to introduce breaking changes without pain. This is a smaller change but makes code verbose.
2. Prepare a new (sub)crate to replicate the mach crate. This is a simpler change but needs more work to maintain.

If the mach-related items aren't updated/removed recently, we could simply de-deprecate them.

[JohnTitor]

cc @rust-lang/libc I'd like to hear your thoughts here.

[JohnTitor]

As the repo owner hasn't responded, I've created https://github.com/JohnTitor/mach2 as a fork.
But I guess we now have some more unstable items, which are likely to be changed in the future, other than mach-related items, so I'm not sure if redirecting to that crate is still the best way here. On the other hand, I would agree that testing with multiple XCode versions is quite heavy though.
I'm going to update the existing deprecation notes to suggest mach2 instead for now, but there are some other mach items that aren't deprecated and we should think of them.

[JohnTitor]

Since mach2 is now used by some Rustaceans and receives PRs, I'd call it an alternative crate for mach. Closing as resolved.