Made "required" error message match "invalid" error message for zip code on Organization name and mailing address

src/registrar/forms/application_wizard.py

@@ -284,6 +284,9 @@ class OrganizationContactForm(RegistrarForm):
                 message="Enter a zip code in the form of 12345 or 12345-6789.",
             )
         ],
+        error_messages={
+            "required": ("Enter a zip code in the form of 12345 or 12345-6789.")
+        },
     )
     urbanization = forms.CharField(
         required=False,