build: upgrade to C++23

Meson 1.1.1 and GCC-13 both support C++23 and a sufficient portion of
the standard has been implemented.  Upgrade the build to leverage it.

Change-Id: Ibb8913a93032d388399b65904013e284780dad70
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/subprojects/nemora-postd/meson.build b/subprojects/nemora-postd/meson.build
index 487b7c0..b1c8921 100644
--- a/subprojects/nemora-postd/meson.build
+++ b/subprojects/nemora-postd/meson.build
@@ -19,7 +19,7 @@
   default_options: [
     'warning_level=3',
     'werror=true',
-    'cpp_std=c++20'
+    'cpp_std=c++23'
   ],
 )