systemd: Use name in unit status messages

Closes #19568

See merge request tails/tails!1088

Welcome Screen: Don't allow unlocking with empty passphrase by pressing Enter

Closes #19570

See merge request tails/tails!1091

Upgrade to Linux 6.1.20-1 (stable branch)

Closes #19484 and #19556

See merge request tails/tails!1080

Closes #19570

Currently, unit status messages are logged like this in the journal:

   Started Journal Service.
   Finished Load Kernel Modules.
   Starting Anonymizing overlay network for TCP...

With this change, the messages are logged like this instead:

   Started systemd-journald.service.
   Finished systemd-modules-load.service.
   Starting tor@default.service...

This makes it easier to search for unit status messages in the journal
by pasting the unit name.

Closes #19568

This fixes this:

    cp: cannot create regular file 'config/chroot_local-includes/usr/share/doc/tails/Changelog': No such file or directory

Regression introduced by d246dc0a.

Persistent Storage design docs

Closes #19223

See merge request tails/tails!1035

Those are now tracked in #19565 and #19566.

Reviewer comment: I don't think we should maintain this data, and if
we don't, sooner or later it'll be confusing.

Reviewer comment: This breaks a bunch of internal wiki links and URLs
passed as Documentation=. The benefit of the renaming does not seem
worth finding & fixing all those.

Test suite: Log when video capture is started/ended

See merge request tails/tails!1083

Closes: #19484

Fix progress information inconsistencies when resuming an automatic upgrade download

Closes #18435

See merge request tails/tails!1071

This reverts commit c4fda5ac,
which actually does not fix the problem.

I need this output for a script which replays videos along with the
corresponding logs from the debug.log.