Move to feature flag to guard for deduction guides

[miscco]

We are currently guarding the deduction guides by standard year but we should just use the feature flag

          Question: doesn't this fail for compilers with partial C++17 support and `-std=c++1z`?

Suggestion: test for the feature instead:

#ifdef __cpp_deduction_guides
template <class _T1, class _T2>
_CCCL_HOST_DEVICE pair(_T1, _T2) -> pair<_T1, _T2>;
#endif // __cpp_deduction_guides

Originally posted by @bernhardmgruber in https://github.com/NVIDIA/cccl/pull/1643#discussion_r1589354895