manuals: Align our tree with upstream C23 macro

Switch all instances of the -isoC-2024 macro to -isoC-2023 selected by
upstream. Keep -isoC-2024 defined, but deprecated, for backwards compat.

MFC after:      3 days (I will resolve merge conflicts with cdefs)

contrib/mandoc/st.c

@@ -56,6 +56,8 @@ LINE("-isoC-tcor1",	"ISO/IEC 9899/TCOR1:1994 (\\(lqISO\\~C90, Technical Corrigen
 LINE("-isoC-tcor2",	"ISO/IEC 9899/TCOR2:1995 (\\(lqISO\\~C90, Technical Corrigendum 2\\(rq)")
 LINE("-isoC-99",	"ISO/IEC 9899:1999 (\\(lqISO\\~C99\\(rq)")
 LINE("-isoC-2011",	"ISO/IEC 9899:2011 (\\(lqISO\\~C11\\(rq)")
+LINE("-isoC-2023",	"ISO/IEC 9899:2024 (\\(lqISO\\~C23\\(rq)")
+/* the -isoC-2024 macro is deprecated in favor of upstream -isoC-2023 */
 LINE("-isoC-2024",	"ISO/IEC 9899:2024 (\\(lqISO\\~C23\\(rq)")
 LINE("-iso9945-1-90",	"ISO/IEC 9945-1:1990 (\\(lqPOSIX.1\\(rq)")
 LINE("-iso9945-1-96",	"ISO/IEC 9945-1:1996 (\\(lqPOSIX.1\\(rq)")

lib/libc/string/memccpy.3

@@ -80,7 +80,7 @@ The
 function conforms to
 .St -p1003.1-2004
 and
-.St -isoC-2024 .
+.St -isoC-2023 .
 .Sh HISTORY
 The
 .Fn memccpy

lib/libc/string/memset.3

@@ -141,4 +141,4 @@ conforms to
 K.3.7.4.1.
 .Fn memset_explicit
 conforms to
-.St -isoC-2024 .
+.St -isoC-2023 .

share/man/man9/cdefs.9

@@ -319,7 +319,7 @@ to distinguish it from newer standards.
 Support for this compilation environment is dependent on compilers supporting
 this configuration.
 Most of the old forms of C have been deprecated or removed in
-.St -isoC-2024 .
+.St -isoC-2023 .
 Compilers make compiling in this mode increasingly difficult and support for it
 may ultimately be removed from the tree.
 .It St -ansiC
@@ -349,7 +349,7 @@ since there are no new C17 only symbols or macros.
 .Pp
 This version of the standard did not introduce any new features, only made
 minor, technical corrections.
-.It St -isoC-2024
+.It St -isoC-2023
 .Dv __STDC_VERSION__ = 202311L
 Strict environment selected with
 .Dv _C23_SOURCE
@@ -385,7 +385,7 @@ implements this as a NOP because too much software breaks with the correct stric
 .It Dv _ANSI_SOURCE Ta St -ansiC
 .It Dv _C99_SOURCE Ta St -isoC-99
 .It Dv _C11_SOURCE Ta St -isoC-2011
-.It Dv _C23_SOURCE Ta St -isoC-2024
+.It Dv _C23_SOURCE Ta St -isoC-2023
 .It Dv _BSD_SOURCE Ta Everything, including Fx extensions
 .El
 .Pp
@@ -406,7 +406,7 @@ Likewise, when C23 dialog is selected with
 or
 .St -p1003.1-2024 ,
 definitions for
-.St -isoC-2024
+.St -isoC-2023
 are also included.
 .Ss Header Visibility Macros
 These macros are set by
@@ -432,7 +432,7 @@ Possible values include 1990, 1999, 2011, 2017 and 2023 for
 .St -isoC-99 ,
 .St -isoC-2011 ,
 ISO/IEC 9899:2018 ("ISO C17"), and
-.St -isoC-2024 ,
+.St -isoC-2023 ,
 respectively.
 .It Dv __BSD_VISIBLE Ta 1 if the
 .Fx