Add note that ufw removal OSSEC alerts might be confusing

Because of an upstream OSSEC bug[1], it alerts that the "ufw"
package was installed right before it alerts that it was removed.

Despite the notification, there is nothing for admins to do, so add
a small note about it.

[1] ossec/ossec-hids#2141

docs/admin/maintenance/noble_migration_prep.rst

@@ -81,6 +81,10 @@ To address it, you can run:
 
 If that emits an error, please send it to :ref:`Support <getting_support>`.
 
+.. note:: You may receive an OSSEC alert that the ``ufw`` package was installed
+   and then removed; it is a known bug and safe to ignore unless the check script
+   continues to alert you about ``ufw``.
+
 Free space
 ----------