[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>

searx/settings.yml

@@ -110,9 +110,10 @@ server:
     X-Robots-Tag: noindex, nofollow
     Referrer-Policy: no-referrer
 
-redis:
-  # URL to connect redis database. Is overwritten by ${SEARXNG_REDIS_URL}.
-  # https://docs.searxng.org/admin/settings/settings_redis.html#settings-redis
+valkey:
+  # URL to connect valkey database. Is overwritten by ${SEARXNG_VALKEY_URL}.
+  # https://docs.searxng.org/admin/settings/settings_valkey.html#settings-valkey
+  # url: valkey://localhost:6379/0
   url: false
 
 ui:
@@ -1809,10 +1810,10 @@ engines:
     shortcut: rt
     disabled: true
 
-  # Required dependency: redis
-  # - name: myredis
+  # Required dependency: valkey
+  # - name: myvalkey
   #   shortcut : rds
-  #   engine: redis_server
+  #   engine: valkey_server
   #   exact_match_only: false
   #   host: '127.0.0.1'
   #   port: 6379