app: parse the release date for the last release.

I was mistakenly using the date of the last check, not the release date.

(cherry picked from commit d5febf6e62)

app/dialogs/about-dialog.c

@@ -286,7 +286,7 @@ about_dialog_add_update (GimpAboutDialog *dialog,
   g_list_free (children);
 
   /* The preferred localized date representation without the time. */
-  datetime = g_date_time_new_from_unix_local (config->check_update_timestamp);
+  datetime = g_date_time_new_from_unix_local (config->last_release_timestamp);
   date = g_date_time_format (datetime, "%x");
   g_date_time_unref (datetime);