[Bug c++/113920] Make -std=gnu++20 default for GCC 15

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

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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   Target Milestone|---                         |15.0