[boost] [1.77][multi_index] post-beta merge request

Hi, I'd like to merge this commit (only testing code and docs affected): https://github.com/boostorg/multi_index/commit/3dad7f38a44783c297b2d477cba21... This is a trivial change for C++23 forward compatibility (and C++<23 UB avoidance), as explained in https://github.com/boostorg/multi_index/issues/45 Best, Joaquín M López Muñoz