doc: mark util.inherits as legacy

Ref: nodejs#38893.

Author: VoltrexKeyva

doc/api/util.md

@@ -406,6 +406,8 @@ changes:
     description: The `constructor` parameter can refer to an ES6 class now.
 -->
 
+> Stability: 3 - Legacy: It is recommended to use ES6 class `extends` instead.
+
 * `constructor` {Function}
 * `superConstructor` {Function}