[mod] migrate from Redis to Valkey

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: Markus Heiser <markus.heiser@darmarit.de>

docs/conf.py

@@ -149,7 +149,7 @@ intersphinx_mapping = {
     "jinja": ("https://jinja.palletsprojects.com/en/stable/", None),
     "linuxdoc" : ("https://return42.github.io/linuxdoc/", None),
     "sphinx" : ("https://www.sphinx-doc.org/en/master/", None),
-    "redis": ('https://redis.readthedocs.io/en/stable/', None),
+    "valkey": ('https://valkey-py.readthedocs.io/en/stable/', None),
 }
 
 issues_github_path = "searxng/searxng"