deb: update debhelper compat level to 13

Now that Ubuntu 20.04 and Debian 10 "buster" are removed, we no longer need
to stay on level 12, and can remove the `override_dh_dwz` that was added in
f9ac2f6 / 5c01d5b

> This also removes the now-obsolete upstart script and updates the
> "debhelper compat level" to 12 (which necessitates adding "override_dh_dwz"
> -- safe to remove once we can update to debhelper-compat 13+).
>
> (See also https://lists.debian.org/debian-devel-announce/2020/02/msg00000.html)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

deb/common/control

@@ -7,7 +7,7 @@ Build-Depends: bash,
                ca-certificates,
                cmake,
                dh-apparmor,
-               debhelper-compat (= 12),
+               debhelper-compat (= 13),
                gcc,
                git,
                libc-dev,