Installing VirtualBox from Sid (since that's the only option now that
it's not been in Debian Stable since Stretch) results in this on
feature/bullseye (and something similar on devel):

The following packages have unmet dependencies:
 virtualbox-guest-utils : Depends: libc6 (>= 2.32) but 2.31-13+deb11u2 is to be installed
 virtualbox-guest-x11 : Depends: libc6 (>= 2.32) but 2.31-13+deb11u2 is to be installed

Last we discussed about this [0] we "decided to do it on a best effort
basis, i.e. to install it from sid when it's possible, and to drop it
when it's not possible anymore".

Seems like the time has come to drop proper Virtualbox.

[0] commit 4e378e52

Installer: make the confirmation dialog more scary

Closes #18301

See merge request tails/tails!593

Switch to squashfs-tools-ng to build IUKs

Closes #18577, #18536, and #18627

See merge request tails/tails!640

Without this package installed, the GitLab CI "test-iuk" test succeeds
but prints this:

    Checking prerequisites...
      test_requires:
        !  Test::LWP::UserAgent is not installed

    ERRORS/WARNINGS FOUND IN PREREQUISITES.  You may wish to install the versions
    of the modules indicated above before proceeding with this installation

    Run 'Build installdeps' to install missing prerequisites.

It does not matter because Test::LWP::UserAgent is only used for tests
that are skipped on GitLab CI, but it did confuse at least one of us:
tails/tails!640 (comment 178808)

refs tails#18577

refs #18577

refs #18577

Move tca developer doc to more appropriate places

Closes #18360

See merge request tails/tails!636

Currently translated at 100.0% (8 of 8 strings)

Currently translated at 100.0% (8 of 8 strings)

Currently translated at 87.5% (7 of 8 strings)

Currently translated at 87.5% (7 of 8 strings)

Currently translated at 75.0% (6 of 8 strings)

Currently translated at 62.5% (5 of 8 strings)

Currently translated at 62.5% (5 of 8 strings)

Currently translated at 50.0% (4 of 8 strings)

Currently translated at 18.1% (2 of 11 strings)

Translation: Tails/wiki/src/home/testing.*.po
Translate-URL: https://translate.tails.boum.org/projects/tails/wikisrchometestingpo/pt/

Currently translated at 95.4% (21 of 22 strings)

Translation: Tails/wiki/src/doc/anonymous_internet/networkmanager/no-wifi.inline.*.po
Translate-URL: https://translate.tails.boum.org/projects/tails/wikisrcdocanonymous_internetnetworkmanagerno-wifiinlinepo/fr/

Currently translated at 100.0% (8 of 8 strings)

Currently translated at 100.0% (73 of 73 strings)