[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>

searx/settings_defaults.py

@@ -185,9 +185,13 @@ SCHEMA = {
         'method': SettingsValue(('POST', 'GET'), 'POST', 'SEARXNG_METHOD'),
         'default_http_headers': SettingsValue(dict, {}),
     },
+    # redis is deprecated ..
     'redis': {
         'url': SettingsValue((None, False, str), False, 'SEARXNG_REDIS_URL'),
     },
+    'valkey': {
+        'url': SettingsValue((None, False, str), False, 'SEARXNG_VALKEY_URL'),
+    },
     'ui': {
         'static_path': SettingsDirectoryValue(str, os.path.join(searx_dir, 'static')),
         'static_use_hash': SettingsValue(bool, False, 'SEARXNG_STATIC_USE_HASH'),