[Libomptarget] 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 that `find_package(CUDA)` is officially deprecated and
can be replaced with `find_package(CUDAToolkit)` instead. This patch
does this and also cleans up a bit of the CMake.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D142568