Bug 18908: Warning "Compatibility levels before 9 are deprecated"

Bug 18908: Warning "Compatibility levels before 9 are deprecated"

The following output:

dh: Compatibility levels before 9 are deprecated (level 7 in use)
   dh_testdir -O--fail-missing
   dh_auto_clean -O--fail-missing
dh_auto_clean: Compatibility levels before 9 are deprecated (level 7 in use)
   dh_clean -O--fail-missing

is given when trying to following these instructions:
https://wiki.koha-community.org/wiki/Building_Debian_Packages_-_The_Easy_Way

This merely tweaks the debian/compat file from 7 to 9.
The message goes away.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
From https://www.debian.org/doc/manuals/maint-guide/dother.en.html:
You may use compat level v9 in certain circumstances for compatibility with older systems. However, using any level below v9 is not recommended and should be avoided for new packages.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>