[support.types] Un-update to refer to C23 7.21

In WG14 N3550, the section number of `<stddef.h>` components has become
7.22 due to addition of `<stdcountof.h>`.

Author: frederick-vs-ja

source/support.tex

@@ -97,7 +97,7 @@
 \ref{support.types.nullptr} and
 \ref{support.types.layout}.
 
-\xrefc{7.22}
+\xrefc{7.21}
 
 \rSec2[cstdlib.syn]{Header \tcode{<cstdlib>} synopsis}
 
@@ -297,7 +297,7 @@
 \tcode{(void*)0}.
 \end{footnote}
 
-\xrefc{7.22}
+\xrefc{7.21}
 
 \rSec2[support.types.layout]{Sizes, alignments, and offsets}
 
@@ -352,7 +352,7 @@
 requirement is supported in every context\iref{basic.align}.
 \tcode{std::is_trivially_default_constructible_v<max_align_t>} is \tcode{true}.
 
-\xrefc{7.22}
+\xrefc{7.21}
 
 \rSec2[support.types.byteops]{\tcode{byte} type operations}