Properly expire ext storage versions (#26601)

* Properly expire ext storage versions

System-wide external storages have no real owner so the current user is
used as owner. However when running cron.php there is no current user,
so no expiry can be done.

This fix adds an user argument to the expire() function to tell for
which user to expire files. This information is anyway always available
now through the expire command job.

* Move version expire setupFS into the expire function

* Add comment about not tearing down in version Storage::expire()

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>