Update for C++ 20 compatibility

[laramiel]

Description

C++ 20 deprecates some std::allocator members. This PR updates xtensor allocators to work with C++20.

https://en.cppreference.com/w/cpp/memory/allocator

[laramiel]

I think that the build checks need to be re-run.

[tdegeus]

Thanks! Looks good to me.

[JohanMabille]

Thanks! and sorry for the late review