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-coturn/tasks/init.yml

@@ -2,6 +2,10 @@
     matrix_systemd_services_list: "{{ matrix_systemd_services_list + ['matrix-coturn.service'] }}"
   when: matrix_coturn_enabled|bool
 
+- set_fact:
+    matrix_systemd_services_list: "{{ matrix_systemd_services_list + ['matrix-coturn-reload.timer'] }}"
+  when: "matrix_coturn_enabled|bool and matrix_coturn_tls_enabled|bool"
+
 # ansible lower than 2.8, does not support docker_image build parameters
 # for self buildig it is explicitly needed, so we rather fail here
 - name: Fail if running on Ansible lower than 2.8 and trying self building