[Bug c++/113920] Make -std=gnu++20 default for GCC 16

[mpolacek at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113920

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Now targeting GCC 16.