Merge topic 'lang-std-c++26'

1ba4a06338 GNU: Add flags for C++26 modes
2038f2c2b1 C++26: Support detection as the default standard

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !9434