Fix __cplusplus version (#2246)

cf https://en.cppreference.com/w/cpp/preprocessor/replace#Predefined_macros
diff --git a/include/avif/avif.h b/include/avif/avif.h
index b3de9ce..c54dd0d 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -48,7 +48,7 @@
 #define AVIF_API
 #endif // defined(AVIF_DLL)
 
-#if defined(AVIF_ENABLE_NODISCARD) || (defined(__cplusplus) && __cplusplus >= 201700L) || \
+#if defined(AVIF_ENABLE_NODISCARD) || (defined(__cplusplus) && __cplusplus >= 201703L) || \
     (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L)
 #define AVIF_NODISCARD [[nodiscard]]
 #else