Direct access to `[AVCaptureDevice devices]` is deprecated since macOS
10.15. An `AVCaptureDeviceDiscoverySession` is to be used instead, which
this PR implements.
When running on older macOS versions or compiling against older platform
SDKs, the deprecated method is used.
PS: The formatting is not proper ObjC style, but that's clang-format's
doing with our current settings.