doc: mark util.inherits as legacy

PR-URL: #38896
Refs: #38893
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>

doc/api/util.md

@@ -404,6 +404,8 @@ changes:
     description: The `constructor` parameter can refer to an ES6 class now.
 -->
 
+> Stability: 3 - Legacy: Use ES2015 class syntax and `extends` keyword instead.
+
 * `constructor` {Function}
 * `superConstructor` {Function}