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.

CHANGELOG.md

@@ -1,3 +1,14 @@
+# 2021-01-14
+
+## Moving from cronjobs to systemd timers
+
+We no longer use cronjobs for Let's Encrypt SSL renewal and `matrix-nginx-proxy`/`matrix-coturn` reloading. Instead, we've switched to systemd timers.
+
+The largest benefit of this is that we no longer require you to install a cron daemon, thus simplifying our install procedure.
+
+The playbook will migrate you from cronjobs to systemd timers automatically. This is just a heads up.
+
+
 # 2021-01-08
 
 ## (Breaking Change) New SSL configuration