update c++23 flag for apple-clang 16

Signed-off-by: Rui Chen <rui@chenrui.dev>

conan/tools/build/flags.py

@@ -257,7 +257,10 @@ def _cppstd_apple_clang(clang_version, cppstd):
         v17 = "c++1z"
         vgnu17 = "gnu++1z"
 
-    if clang_version >= "13.0":
+    if clang_version >= "16.0":
+        v23 = "c++23"
+        vgnu23 = "gnu++23"
+    elif clang_version >= "13.0":
         v20 = "c++20"
         vgnu20 = "gnu++20"
     elif clang_version >= "10.0":