Add missing C++26 insert_or_assign overloads for map and flat_map

[igaztanaga]

In C++26:

template<class K, class M> pair<iterator, bool> insert_or_assign(K&& k, M&& obj);

and

template<class K, class M> iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

where added under __cpp_lib_associative_heterogeneous_insertion >= 202311L