Make HasAttributes::getDateFormat public (#22618)

It doesn't really make sense to make the getter protected function if the setter is public.

src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php

@@ -781,7 +781,7 @@ public function getDates()
      *
      * @return string
      */
-    protected function getDateFormat()
+    public function getDateFormat()
     {
         return $this->dateFormat ?: $this->getConnection()->getQueryGrammar()->getDateFormat();
     }