systemd.example: remove DefaultDependencies=no

The systemd unit has historically always used DefaultDependencies=no.
When only Before=network.target was used, the dependencies (as seen with
'systemctl list-dependencies ufw.service') were:

ufw.service
 |_system.slice

When Before=network.target was changed to Before=network-pre.target and
Wants=network-pre.target, this became:

ufw.service
 |_system.slice
 |_network-pre.target

Removing DefaultDependencies=no (DefaultDependencies defaults to 'yes')
pulls in the sysinit.target which changes this to (on a Debian 11
system):

ufw.service
 |_system.slice
 |_network-pre.target
 |_sysinit.target
   |_apparmor.service
   |_blk-availability.service
   |_dev-hugepages.mount
   |_dev-mqueue.mount
   |_keyboard-setup.service
   |_kmod-static-nodes.service
   |_lvm2-lvmpolld.socket
   |_lvm2-monitor.service
   |_proc-sys-fs-binfmt_misc.automount
   |_sys-fs-fuse-connections.mount
   |_sys-kernel-config.mount
   |_sys-kernel-debug.mount
   |_sys-kernel-tracing.mount
   |_systemd-ask-password-console.path
   |_systemd-binfmt.service
   |_systemd-boot-system-token.service
   |_systemd-hwdb-update.service
   |_systemd-journal-flush.service
   |_systemd-journald.service
   |_systemd-machine-id-commit.service
   |_systemd-modules-load.service
   |_systemd-pstore.service
   |_systemd-random-seed.service
   |_systemd-sysctl.service
   |_systemd-sysusers.service
   |_systemd-timesyncd.service
   |_systemd-tmpfiles-setup-dev.service
   |_systemd-tmpfiles-setup.service
   |_systemd-udev-trigger.service
   |_systemd-udevd.service
   |_systemd-update-utmp.service
   |_cryptsetup.target
     |_systemd-cryptsetup@vda5_crypt.service
   |_local-fs.target
     |_-.mount
     |_boot.mount
     |_systemd-fsck-root.service
     |_systemd-remount-fs.service
   |_swap.target
     |_dev-mapper-debian\x2d\x2dbuster\x2d\x2damd64\x2d\x2dvg\x2dswap_1.swap

While ufw is meant to come up before networking, there is no reason why
it shouldn't come up after 'basic system initialization is
completed'[1]. This should help make ufw startup more robust on systems
that need something from sysinit.

[1]https://www.freedesktop.org/software/systemd/man/systemd.unit.html