Require C++20

[aprokop]

TODO:

• Clang build
  Install LLVM 14 with gcc newer than 9 to get new libstdc++. Probably need to use newer image than ubuntu:18.04, but LLVM does not provide tarballs for any ubuntu other than 18.04.
  It is possible to install clang like this

  wget https://apt.llvm.org/llvm.sh
  chmod +x llvm.sh
  sudo ./llvm.sh 14
  

  But it does not give us full control over versioning.
• Clang+CUDA build
  Need to figure it out. Clang 14 seems to support up to cuda 11.5, and there are weird compilation failures with 12, it seems.
  [CUDA 12] clang error: no template named 'texture' llvm/llvm-project#61340
  It was only fixed in Clang 16

[masterleinad]

What's the motivation to require C++20 at this point?

[aprokop]

What's the motivation to require C++20 at this point?

First of all, I don't yet know if we will. The arguments for it are:

• Kokkos is going to switch in a few months
• We are doing a backwards incompatible change with ArborX 2.0. If we do the switch to c++20 at the same time, we would only break backwards compatibility once