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/defaults/main.yml

@@ -48,7 +48,11 @@ matrix_base_data_path_mode: "750"
 
 matrix_static_files_base_path: "{{ matrix_base_data_path }}/static-files"
 matrix_systemd_path: "/etc/systemd/system"
+
+# This is now unused. We keep it so that cleanup tasks can use it.
+# To be removed in the future.
 matrix_cron_path: "/etc/cron.d"
+
 matrix_local_bin_path: "/usr/local/bin"
 
 matrix_host_command_docker: "/usr/bin/env docker"