Fix return type of iterateKeyValue() (#6402)

|      Q       |   A
|------------- | -----------
| Type         | bug
| Fixed issues | N/A

#### Summary

I messed this up in #6399. We cannot make any assumptions regarding key
and value here.

src/Result.php

@@ -184,7 +184,7 @@ public function iterateAssociative(): Traversable
     }
 
     /**
-     * @return Traversable<int,list<mixed>>
+     * @return Traversable<mixed, mixed>
      *
      * @throws Exception
      */