mark bind9-dnsutils installed manual

Otherwise it will get removed by autoremove and then reinstalled by
Puppet...

howto/upgrades/bullseye.md

@@ -130,6 +130,7 @@ connection.
         export LC_ALL=C.UTF-8 &&
         sudo ttyrec -a -e screen /var/log/upgrade-buster.ttyrec
 
+        apt-mark manual bind9-dnsutils
         apt purge $(dpkg -l | awk '/^rc/ { print $2 }') # purge removed packages
         apt autoremove -y --purge
         apt purge $(deborphan --guess-dummy | grep -v python-is-python2)