#166172 moved the effective minimum deployment target on macOS to 10.15 (because `aligned_alloc` is not defined before that in the C stdlib), and indeed, it was mentioned in that PR that libc++ only supports macOS 11 and later. This PR rectifies the documentation and the code to reflect the actually supported deployment targets on macOS. See [1] for additional discussion about this. [1]: https://discourse.llvm.org/t/minimum-macos-deployment-target-increases-to-11-0-in-v22-1-visibility-discussion-on-update-policy