Replace cronjobs with systemd timers

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/756

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/737

I feel like timers are somewhat more complicated and dirty (compared to
cronjobs), but they come with these benefits:

- log output goes to journald
- on newer systemd distros, you can see when the timer fired, when it
will fire, etc.
- we don't need to rely on cron (reducing our dependencies to just
systemd + Docker)

Cronjobs work well, but it's one more dependency that needs to be
installed. We were even asking people to install it manually
(in `docs/prerequisites.md`), which could have gone unnoticed.

Once in a while someone says "my SSL certificates didn't renew"
and it's likely because they forgot to install a cron daemon.

Switching to systemd timers means that installation is simpler
and more unified.

roles/matrix-base/templates/usr-local-bin/matrix-remove-all.j2

@@ -20,8 +20,6 @@ else
 		rm -f {{ matrix_systemd_path }}/$s
 	done
 	systemctl daemon-reload
-	echo "Remove matrix cronjobs"
-	find /etc/cron.d/ -name "matrix-*" -delete
 	echo "Remove matrix scripts"
 	find {{ matrix_local_bin_path }}/ -name "matrix-*" -delete
 	echo "Remove unused Docker images and resources"