Fixes #13655

This change uses ICU (International Components for Unicode) to improve boundary detection in user search.

This change also adds a new dependency on libicu-dev and pkg-config for the Debian packages, which are available in all supported distros.

The --force flag of dpkg-statoverride has been deprecated (apparently starting
with the dpkg version in Debian buster). It offers --force-all as q quick fix,
but the usage in the Debian postinst script is probably covered by
--force-statoverride-add.

Fixes: #8391

Signed-off-by: Jörg Behrmann <behrmann@physik.fu-berlin.de>

* Update debian packaging to debhelper version 12

Don't call dh_installinit anymore, because it has been deprecated, and use
dh_installsystemd instead of dh_systemd_enable for the same reason.

Signed-off-by: Jörg Behrmann <behrmann@physik.fu-berlin.de>

* Drop preinst script

It was used for reasons of interactions of dh_systemd_start and dh_installinit,
which have both be deprecated

Signed-off-by: Jörg Behrmann <behrmann@physik.fu-berlin.de>

* Drop /etc/default file

It was no longer being installed.

* Remove debian/compat file

This is managed by the control file nowadays

Otherwise the files of the synapse user are readable by the nobody user, which
is unsafe.

Signed-off-by: Jörg Behrmann <behrmann@physik.fu-berlin.de>