pg_upgrade errors out if distributed deadlock detector fires on new server

Within our upgrade documentation we stated 'stop old or new servers' as the third step. If we run the new server before running the pg_upgrade, distributed deadlock daemon also starts. Even if we stop the new server before running pg_upgrade, upgrade process errors out since the daemon cannot read pg_index table. So, we may add a warning about this issue similar to the warning about creating the extension on step 3 of upgrading coordinator node.