Werk #14733: cmk-update-agent: Prevent the minutely timer from flooding the journalctl on Linux

The agent updater comes with a systemd timer unit that invokes cmk-update-agent minutely in order to check for a pending installation.

We now prevent the timer (or more precisely: the corresponding service unit) from writing status messages on every start and stop to journalctl.

A detailed log can already be found at /var/lib/check_mk_agent/cmk-update-agent.log

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