Tests: Don't unnecessarily set CUDA_ARCHITECTURES
Architecture 30 was removed with CUDA 11, so most of the CUDA tests fail with
it.
Remove setting the architecture and bump the minimum version to 3.18, so
CMP0104 takes effect and we can rely on the default architecture, which is
guaranteed to be compilable.
Testing the functionality of CUDA_ARCHITECTURES
is already covered by
CudaOnly.Architecture
and CudaOnly.CompileFlags
.