[ci skip] Add json_validate() RFC link to UPGRADING

Author: TimWolla

UPGRADING

@@ -58,6 +58,7 @@ PHP 8.3 UPGRADE NOTES
 - JSON:
   . Added json_validate(), which returns whether the json is valid for
     the given $depth and $options.
+    RFC: https://wiki.php.net/rfc/json_validate
 
 - Posix:
   . Added posix_sysconf call to get runtime informations.