Update countBy docblock in Enumerable interface to allow for enum callback (#56897)

Author: jnoordsij

src/Illuminate/Collections/Enumerable.php

@@ -1222,7 +1222,7 @@ public function count(): int;
     /**
      * Count the number of items in the collection by a field or using a callback.
      *
-     * @param  (callable(TValue, TKey): array-key)|string|null  $countBy
+     * @param  (callable(TValue, TKey): array-key|\UnitEnum)|string|null  $countBy
      * @return static<array-key, int>
      */
     public function countBy($countBy = null);