This PR bumps the project's C++ language standard from C++17 to C++20
(host, CUDA, and HIP) and removes support for CUDA 11, which cannot
compile C++20. CI matrices are updated to drop CUDA 11.8 and add CUDA
13.0 alongside 12.9.
- C++20 enables newer language features across the codebase.
- C++20 requires CUDA ≥ 12.0, so CUDA 11.8 can no longer be supported.
- Blackwell / sm_100 targets require CUDA ≥ 12.8, so CUDA 13.x is added
to CI coverage.