Switch to C++20 and replace std::enable_if with template constraints

It's about time we switch to C++20. I believe the only compiler that Walberla still supports that doesn't support C++20 is nvcc from CUDA < 12, but there is no reason to run anything that old anymore: The newest CUDA driver that only supported CUDA 11 is well past any long-term support and has plenty of known security vulnerabilities.

One C++20 feature I have come to appreciate is template constraints (i.e. the requires keyword) which can replace std::enable_if. It makes the code more legible and leads to much nicer error messages when you try to use a disabled function/class.

Before this merge request can be merged, some build images will need to be updated from CUDA 11 to CUDA 12. Depends on !739 (merged).