Move to C++-20

[mkstoyanov]

The C++-20 standard has good features:

• concepts allow for better template overloads
• build in variables for things like sqrt(3) and pi
• simple usage of ranges algorithms, no need for .begin() .end()
• span that can eventually replace view and ssize that can replace the static conversions to unsigned

The main consideration is support:

• ubuntu 22.04 and the latest 24.04 have full support, which mean all of our workstations
• apple supports it

Question:

• What about OLCF Frontier and similar large machines of interest?

[quantumsteve]

We may also want to bump the CUDA standard to 17 or 20.
https://github.com/project-asgard/asgard/blob/develop/CMakeLists.txt#L398

[mkstoyanov]

My CUDA kernels are filled with if constexpr statements, CUDA must be defaulting to 17.

I don't know if CUDA 20 will work, there may be issues there as 20 was a big change and CUDA is usually behind. But we should bump CUDA to 17, saying 14 in CMake is technically a bug.

[mkstoyanov]

Reconsider updating the C++ standard post 1.0 release.