Replace boolean definition with C23 bool

Completely fixes all of the C23 related compilation errors instead of only fixing GCC15 like !2707 (merged) did but is a more invasive solution in comparison.

Uses C23's native bool type when available and <stdbool.h> otherwise for backwards compat.

(Note: the oldstable pipeline script appears to be broken atm, so I can't do anything about that one failing.)