Werk #12986: apt: Handle Security Updates Line

Newer versions of the Linux command apt display something like "10 standard security updates" or "3 esm-infra security updates" if security updates are available. Up to now, the corresponding check plugin apt was not able to handle this line. If such a line was present, the service "APT Updates" was not discovered or went stale if already discovered.

This has been fixed.

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