Consistently use ::cuda::std::iterator_traits in CCCL

[bernhardmgruber]

Since #3837 is merged, we can now replace all uses of any iterator_traits by ::cuda::std::iterator_traits. Any derived trait (like cub::detail::value_t) should also be based on ::cuda::std::iterator_traits.

thrust::iterator_traits should be replaced by a deprecated alias so users can no longer specialize it. We should also add a comment that users should specialize std::iterator_traits or ::cuda::std::iterator_traits instead.