meson: update C++ standard to C++23

C++ is evolving faster than C, so it's useful to enable new standards,
especially for standard library.
Update to most recent standard available in our build environments.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260124182921.531562-11-pierrick.bouvier@linaro.org
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>