CUDA 12 on Linux with -std=c++20 compile error

[Robin Söderholm]

Describe the bug
Using nvcc with -std=c++20 gives a compile error in taskflow's cuda_memory.hpp due to usage of std::allocator<void>::const_pointer which do no longer exist (std::allocator<void>-specialisation is deprecated in c++17 and removed in c++20)

To Reproduce
Steps to reproduce the behavior:

1. Get CUDA Toolkit 12
2. Try to create some CuFlow objects
3. Press compile
4. See error

Desktop (please complete the following information):

• OS: ALMA Linux

[Tsung-Wei Huang]

Thank you for the fix! I have merged your pull request :)