manuals: Align our tree with upstream C23 macro

Switch all instances of the -isoC-2024 macro to -isoC-2023 selected by
upstream. Keep -isoC-2024 defined, but deprecated, for backwards compat.

MFC after: 3 days (resolved merge conflicts w/ cdefs and memccpy.3)

(cherry picked from commit 1b6936e125382523509901a4fc5b2932ed0e742d)