Use std::less as a default comparator instead of operator< for additional algorithms

[dmitriy-sobolev]

Summary:
C++ standard replaces operator< as a default comparator for many algorithms with std::less since c++20 (see std::sort as an example).
Let's do the same replacement for the additional algorithms. std::less is a more genetic alternative to operator<, thus it can be done even for c++17 mode.

The issue is motivated by this discussion: #1692 (comment).

URLs:
additional_algorithms.rst