Rename c2x / gnu2x tests to c23 / gnu23

Complete the internal renaming from "C2X" and related names in GCC by
renaming *-c2x and *-gnu2x tests to *-c23 and *-gnu23.

Tested for x86_64, and with build-many-glibcs.py for powerpc64le.

stdio-common/tst-scanf-binary-c23.c

@@ -0,0 +1,31 @@
+/* Test scanf functions with C23 binary integers (narrow strings,
+   no extensions).
+   Copyright (C) 2022-2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* Some versions of GCC supported for building glibc do not support
+   -std=c23 or -std=c2x.  */
+#undef _GNU_SOURCE
+#define _ISOC23_SOURCE
+
+#define CHAR char
+#define FNW
+#define L_(C) C
+#define TEST_C23 1
+#define STD "c23"
+
+#include <tst-scanf-binary-main.c>