revert changes to old() helper (#56769)

this is a breaking change, as previously users could pass any type of value to `$default`. with this change it forces all `$default`s to be `string|array|null`.

Author: browner12

src/Illuminate/Foundation/helpers.php

@@ -630,8 +630,9 @@ function now($tz = null): CarbonInterface
      *
      * @param  string|null  $key
      * @param  \Illuminate\Database\Eloquent\Model|string|array|null  $default
+     * @return string|array|null
      */
-    function old($key = null, $default = null): string|array|null
+    function old($key = null, $default = null)
     {
         return app('request')->old($key, $default);
     }