Update old() docblock (#54641)

This simply copies the latest docblock from the wrapped method (`InteractsWithFlashData::old()`)

Author: AJenbo

src/Illuminate/Foundation/helpers.php

@@ -604,8 +604,8 @@ function now($tz = null)
  * Retrieve an old input item.
  *
  * @param string|null $key
- * @param mixed $default
- * @return mixed
+ * @param \Illuminate\Database\Eloquent\Model|string|array|null $default
+ * @return string|array|null
  */
  function old($key = null, $default = null)
  {