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>

A script to build & install a simple & isolated redis service, dedicated to
SearXNG and connected via Unix socket.

    $ ./manage redis.help
    redis.:
      devpkg    : install essential packages to compile redis
      build     : build redis binaries at /800GBPCIex4/share/SearXNG/dist/redis/6.2.6/amd64
      install   : create user (searxng-redis) and install systemd service (searxng-redis)
      remove    : delete user (searxng-redis) and remove service (searxng-redis)
      shell     : start bash interpreter from user searxng-redis
      src       : clone redis source code to <path> and checkput 6.2.6
      useradd   : create user (searxng-redis) at /usr/local/searxng-redis
      userdel   : delete user (searxng-redis)
      addgrp    : add <user> to group (searxng-redis)
      rmgrp     : remove <user> from group (searxng-redis)

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>