[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>

.devcontainer/Dockerfile

@@ -1,4 +1,4 @@
 FROM mcr.microsoft.com/devcontainers/base:debian
 
 RUN apt-get update && \
-    apt-get -y install python3 python3-venv redis firefox-esr graphviz imagemagick librsvg2-bin fonts-dejavu shellcheck
+    apt-get -y install python3 python3-venv valkey firefox-esr graphviz imagemagick librsvg2-bin fonts-dejavu shellcheck