Give tails-synchronize-data-to-new-persistent-volume-on-shutdown.service more time to do its job (refs: #17278)

We currently set DefaultTimeoutStopSec=5s, which means copying just the APT
lists (215MB currently), not even taking into account the actual additional
packages installed by the user, would need to happen at no less than 43MB/s for
the ExecStop= to succeed.

Most real-world bare metal USB sticks don't provide anything close
to this write rate.

I can't reproduce the problems caused by this bug when running the test suite on
my laptop, but it does happen regularly on lizard: even virtual USB stick backed
by tmpfs are not always fast enough there.