[9.x] Allow jsonSerialize() to return other types (#42133)

* Update jsonSerialized() return to type to mixed

* Update testJsonSerialize test

* Update to use a more specific type: array|string vs. mixed

* Typo - missing bracket

* Updated Collection's jsonSerialize return type to be array

* Revert EnumeratesValues' jsonSerialize() type

* Revert to mixed type

Eloquent/Model.php

@@ -1539,9 +1539,9 @@ public function toJson($options = 0)
     /**
      * Convert the object into something JSON serializable.
      *
-     * @return array
+     * @return mixed
      */
-    public function jsonSerialize(): array
+    public function jsonSerialize(): mixed
     {
         return $this->toArray();
     }