Demote cron log entries from "notice" to "info".

Cron currently logs its results as notices:

    $this->logger->notice('Cron run completed.');
        $logger->notice('Starting execution of @module_cron().', [
        $logger->notice('Starting execution of @module_cron(), execution of @module_previous_cron() took @time.', [
      $logger->notice('Execution of @module_previous_cron() took @time.', [

The RFC says this is for "significant" conditions (Wikipedia says "events that are unusual but not error conditions").

I think the Informational level would be more appropriate here, since the cron run is a routine event that doesn't normally require attention.