Bump the minimum CMake version to 3.18, as they have in PyTorch?

[joewallwork]

          Do you think it might be worth bumping the minimum CMake version to 3.18, as they have in [PyTorch](https://github.com/pytorch/pytorch/blob/main/torch/CMakeLists.txt)?

I think 3.15 still works in most cases, but CMake 3.18 was the first version to include support for any compiler, enabling CMAKE_CUDA_STANDARD 17 support (it also includes potentially useful things like CMAKE_CUDA_ARCHITECTURES), so it may be safer, although C++17 is already in the FTorch requirements separately.

Originally posted by @ElliottKasoar in https://github.com/Cambridge-ICCS/FTorch/issues/233#issuecomment-2587865382