Mark atty as officially unmaintained

[BlackHoleFox]

Should be fairly uncontroversial as it explicitly meets the unmaintained policy's of explicit:

Explicitly unmaintained: a crate's author has declared that they are no longer maintaining a particular crate.

The author of atty published this README update making the maintenance status official:

This is crate is no longer maintained and users are encouraged to prefer the equivalant functionality of [std.io.IsTerminal]...

Since the previous maintainer was clear about alternatives, I made them more explicit in the advisory instead of the usual "these have not been looked and might not work, etc".

Closes #2087

[BlackHoleFox]

cc @pinkforest since you opened the original issue upstream asking about the crate's maintenance status.