[forward.iterators] Use "Cpp17" requirement (#6612)

Author: frederick-vs-ja

source/iterators.tex

@@ -2214,7 +2214,7 @@
 \pnum
 A class or pointer type
 \tcode{X}
-meets the requirements of a forward iterator if
+meets the \oldconcept{ForwardIterator} requirements if
 \begin{itemize}
 \item \tcode{X} meets the \oldconcept{InputIterator} requirements\iref{input.iterators},