build: improve development build flags

Use c++23 for better warnings / checks (but only for local 'hacker'
builds)

Makefile

@@ -35,13 +35,13 @@ endif
 
 # when MU_HACKER is set, do a debug build
 # MU_HACKER is for djcb & compatible developers
-# note that mu uses C++17, we only pass C++20 here
+# note that mu uses C++17, we only pass C++23 here
 # for the better error messages (esp. for fmt).
 ifneq (${MU_HACKER},)
 MESON_FLAGS:=$(MESON_FLAGS) '-Dbuildtype=debug'    \
 			    '-Db_sanitize=address' \
 			    '-Dreadline=enabled'   \
-			    '-Dcpp_std=c++20'
+			    '-Dcpp_std=c++23'
 endif
 
 .PHONY: all build-valgrind