[boost] [multi_index] upgrade to C++11 and potential backwards compatibility issues

Hi, Boost.MultiIndex has been upgraded to require C++11 a minimum, and in the process the library has been refactored to no longer use Boost.MPL and Boost.Preprocessor. The changes, benefits and potential backwards compatibility issues are explained in this external article: https://bannalia.blogspot.com/2025/12/boostmultiindex-refactored.html If you're a Boost.MultiIndex user please test this upgraded version in advance of the target Boost 1.91 release (develop branch of boostorg/multi_index) so that we have time to fix any problem that may arise. Thank you! Best, Joaquín M López Muñoz