[modernize] Use std::is_scalar_v in jet_traits.h

Change-Id: Iace518cd4b32b96cf7b7adb64b7695064ae1d04d

include/ceres/internal/jet_traits.h

diff --git a/include/ceres/internal/jet_traits.h b/include/ceres/internal/jet_traits.h
index f504a61..a6c1e1f 100644
--- a/include/ceres/internal/jet_traits.h
+++ b/include/ceres/internal/jet_traits.h
@@ -85,7 +85,7 @@
 
 // The rank of a scalar is 0.
 template <typename T>
-struct Rank<T, std::enable_if_t<std::is_scalar<T>::value>>
+struct Rank<T, std::enable_if_t<std::is_scalar_v<T>>>
     : std::integral_constant<int, 0> {};
 
 // The rank of a Jet is given by its dimensionality.