[!356] [C++20] use `std::allocator_traits`
Merge branch 'gcc10' into 'master'
ref:core/dune-istl Several members of std::allocator were already deprecated in C++17 and have now been removed in C++20. This patch uses the appropriate members of std::allocator_traits instead.
See merge request core/dune-istl!356
(cherry picked from commit a9a901a7)
54c17e51 [C++20] use std::allocator_traits