[mod] migrate from Redis to Valkey (#4795)

This patch migrates from `redis==5.2.1` [1] to `valkey==6.1.0` [2].

The migration to valkey is necessary because the company behind Redis has decided
to abandon the open source license. After experiencing a drop in user numbers,
they now want to run it under a dual license again. But this move demonstrates
once again how unreliable the company is and how it treats open source
developers.

To review first, read the docs::

    $ make docs.live

Follow the instructions to remove redis:

- http://0.0.0.0:8000/admin/settings/settings_redis.html

Config and install a local valkey DB:

- http://0.0.0.0:8000/admin/settings/settings_valkey.html

[1] https://pypi.org/project/redis/
[2] https://pypi.org/project/valkey/

Co-authored-by: HLFH <gaspard@dhautefeuille.eu>
Co-authored-by: Markus Heiser <markus.heiser@darmarit.de>

utils/templates/etc/apt/sources.list.d/ubuntu-stable-backports.sources

@@ -0,0 +1,6 @@
+Types: deb deb-src
+URIs: http://us.archive.ubuntu.com/ubuntu/
+Suites: ${DIST_VERSION_CODENAME}-backports
+Components: main multiverse restricted universe
+Enabled: yes
+Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg