mac-capture: Use system CGDisplayStream.h header on macOS 14 and newer

ee3c2d0 introduced a local copy of the CGDisplayStream.h header as the
macOS 13.3 SDK erroneously declared functions as unavailable on older
systems. This has been fixed in the macOS 14 SDK with the functions
correctly being declared as available on 10.8, so we can use the system
header again, and in the future remove the local copy once our minimum
compile version is new enough.