Ubuntu: Bump to 20.04.4

Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/Ubuntu/Ubuntu 20.04 Root on ZFS.rst

@@ -149,8 +149,8 @@ Caution
 System Requirements
 ~~~~~~~~~~~~~~~~~~~
 
-- `Ubuntu 20.04.3 (“Focal”) Desktop CD
-  <https://releases.ubuntu.com/20.04/ubuntu-20.04.3-desktop-amd64.iso>`__
+- `Ubuntu 20.04.4 (“Focal”) Desktop CD
+  <https://releases.ubuntu.com/20.04/ubuntu-20.04.4-desktop-amd64.iso>`__
   (*not* any server images)
 - Installing on a drive which presents 4 KiB logical sectors (a “4Kn” drive)
   only works with UEFI booting. This not unique to ZFS. `GRUB does not and