Apple dropped support for some older platforms, so we can also remove
the annotations for them.
See https://developer.apple.com/support/xcode/ for the supported
versions.
There are discussions about different ways of implementing `stop_token` to make it more performant
mark `stop_token` as experimental to allow us to change the design before it is shipped
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
Differential Revision: https://reviews.llvm.org/D154700
