[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>

docs/admin/settings/settings_valkey.rst

@@ -0,0 +1,53 @@
+.. _settings valkey:
+
+===========
+``valkey:``
+===========
+
+.. _Valkey:
+    https://valkey.io
+.. _Valkey-Installation:
+    https://valkey.io/topics/installation/
+.. _There are several ways to specify a database number:
+    https://valkey-py.readthedocs.io/en/stable/connections.html#valkey.Valkey.from_url
+
+A Valkey_ DB can be connected by an URL, in section :ref:`valkey db` you will
+find a description to test your valkey connection in SearXNG.
+
+``url`` : ``$SEARXNG_VALKEY_URL``
+  URL to connect valkey database. `There are several ways to specify a database
+  number`_::
+
+    valkey://[[username]:[password]]@localhost:6379/0
+    valkeys://[[username]:[password]]@localhost:6379/0
+    unix://[[username]:[password]]@/path/to/socket.sock?db=0
+
+  When using sockets, don't forget to check the access rights on the socket::
+
+    ls -la /usr/local/searxng-valkey/run/valkey.sock
+    srwxrwx--- 1 searxng-valkey searxng-valkey ... /usr/local/searxng-valkey/run/valkey.sock
+
+  In this example read/write access is given to the *searxng-valkey* group.  To
+  get access rights to valkey instance (the socket), your SearXNG (or even your
+  developer) account needs to be added to the *searxng-valkey* group.
+
+
+.. _Valkey Developer Notes:
+
+Valkey Developer Notes
+======================
+
+To set up a local Valkey_ DB, set the URL connector in your YAML setting:
+
+.. code:: yaml
+
+   valkey:
+     url: valkey://localhost:6379/0
+
+To install a local Valkey_ DB from package manager read `Valkey-Installation`_
+or use:
+
+.. code:: sh
+
+   $ ./utils/searxng.sh install valkey
+   # restart your SearXNG instance