refactor(conf): Update Ubuntu default package strings

apt-zfs-snapshot.conf

@@ -3,7 +3,7 @@ dry_run='false'
 
 # Exact package-name regular expression (without the surrounding ^ and $) for
 # packages whose snapshots use the important retention chain.
-important_names='linux-image(-.*)?|linux-headers(-.*)?|systemd|zfs-(dkms|utils)'
+important_names='linux-(firmware|headers|image)(-.*)?|systemd|zfs'
 
 # Number of snapshots retained in each chain.
 snapshots_trivial_keep='25'