[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_server.rst

@@ -36,7 +36,7 @@
 
 ``limiter`` :  ``$SEARXNG_LIMITER``
   Rate limit the number of request on the instance, block some bots.  The
-  :ref:`limiter` requires a :ref:`settings redis` database.
+  :ref:`limiter` requires a :ref:`settings valkey` database.
 
 .. _public_instance: