[FEA] Use mdspan from CCCL

[cjnolet]

This feature has been a long time coming, but when we initially set out to start using mdspan, the CCCL didn’t yet have support for it, which caused us to ship out own copy of the reference implementation.

Now that CCCL has support for mdspan, we should include libcu++ as a dependency, update our infrastructure to use that, and remove the copy of the Kokkos version from our include/thirdparty directory.