Simplify the customization for Kokkos backend
When using Kokkos, some Kokkos devices/backends require enabling a
language (such as CUDA or HIP). Previously, this required users to set a
variable named VTK_KOKKOS_BACKEND
to an appropriate value. This has
been simplified by getting the Kokkos backends directly from the Kokkos
configuration so that the compilers are set up automatically.