[master] deb: use debhelper-compat=11 for compatibility with ubuntu 18.04

[thaJeztah]

Commit f9ac2f6 (#686) updated the "debhelper compat
level" to 13 (12?), which seems to have introduced a regression when executing the
preRM and postInst scripts;

invoke-rc.d: syntax error: unknown option "--skip-systemd-native"
   dpkg: error processing package docker-ce (--install):
   installed docker-ce package post-installation script subprocess returned error exit status 1

This patch tries to downgrade it to 11, which looks to be the default for
ubuntu 18.04 "bionic"

[thaJeztah]

Oh, the fun, so: https://manpages.ubuntu.com/manpages/focal/man7/debhelper.7.html#compatibility%20levels

v11 This mode is discouraged.

   The compat 11 is discouraged for new packages as it suffers from feature interaction
   between dh_installinit and dh_installsystemd causing services to not run correctly in
   some cases.  Please consider using compatibility mode 10 or 12 instead.  More details
   about the issue are available in Debian#887904 and
   <https://lists.debian.org/debian-release/2019/04/msg01442.html>.

And

 v12 This is the recommended mode of operation.

So, that's all fun and nice, but 18.04 (which is an LTS release) cannot use the recommended version, and only supports a version thats discouraged ????

[thaJeztah]

Please consider using compatibility mode 10 or 12 instead. 

So this updates it to 11 (previously we were using 10), but looks like that's not really a good option.

Perhaps we can make it conditional (use 10 for bionic, and 12 for everything above) 🤔

[thaJeztah]

Wondering if we need #696 as well (assuming that the "depends" is updated according to the compat-level)