Update MPS docs to reflect latest runtime changes (#1842)

Summary:
Update MPS docs to reflect latest runtime changes for iOS and macOS support.
- Note that for lowering there is no dependency on a specific macOS version. Only the runtime needs macOS12+/iOS15+ to work with the lowered module.

(cc shoumikhin, cccclai)

Pull Request resolved: https://github.com/pytorch/executorch/pull/1842

Reviewed By: shoumikhin

Differential Revision: D53453496

Pulled By: cccclai

fbshipit-source-id: 6f38693259e4d69dc1059f742567f798409038e4