Tails 3.15 apt config references tor-0.4.0.x-experimental packages, which are no longer available

Tails 3.15 apt sources lists include a reference to tor-0.4.0-experimental components, which are no longer available on the Tor Project repos. This breaks `sudo apt-get update`, and automatic installation of packages from the Persistent volume.

Steps to Reproduce

Start Tails 3.15 with an admin password.
In a terminal, run sudo apt-get update

Expected Behavior

sudo apt-get update completes without error.

Actual Behavior

sudo apt-get update fails with error:

---

Fetched 30.1 MB in 5min 13s (96.0 kB/s)
Reading package lists… Done
W: The repository ‘tor+http://sdscoq7snqtznauu.onion/torproject.org tor-experimental-0.4.0.x-stretch Release’ does not have a Release file.
N: Data from such a repository can’t be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch tor+http://sdscoq7snqtznauu.onion/torproject.org/dists/tor-experimental-0.4.0.x-stretch/main/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
—-

This is similar to issue #15978 (closed), which affected users of SecureDrop in particular but also anyone installing custom packages via apt.

:sajolida: is watching this.

Feature Branch: bugfix/16883-drop-tor-0.4.0.x-experimental+force-all-tests

Related issues

• Related to #16931 (closed)
• Related to #16790 (closed)
• Related to #15978 (closed)
• Blocks #16209

Original created by @kogorman on 16883 (Redmine)