btrbk supports hourly backups, so it makes sense for the timer to run hourly,
too.
Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Using `WantedBy=multi-user.target` makes boot wait for btrbk.service
before it's considered "finished". This can be checked by running
`systemd-analyze` or checking the system log using `journalctl`.
Timers should use the "timers.target" target, see systemd.special(7).