sysupdate: automatically clean up orphaned files after auto-update (#42714)

This adds an operation equivalent to "systemd-sysupdate cleanup" after
an update completed (regardless if that update was entirely successful
or not). This ensures that any orphaned files are automatically cleaned
up, if they are not referenced by any transfer file's patterns anymore.

Follow-up for: d82e256bb9d151b185a8afec1fcacd8fbe80555c