refactor: remove references to deprecated values under std::allocator

Summary:
Includes allocator::pointer, allocator::const_pointer, allocator::reference and allocator::const_reference which are deprecated in c++17 and removed in c++20. See https://en.cppreference.com/w/cpp/memory/allocator

Also prefer using over typedef see: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rt-using

This is a backport of core#23181

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15313