[nvptx-arch] Remove `find_package(CUDA)` as it has been deprecated.

Since D137724 and the LLVM 17 release we have updated to CMake version
3.20. This means we can use `find_package(CUDAToolkit)` instead as the
legacy module is deprecated.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D142570