Make SearXNG use valkey since it is installed

[mikkotor]

Pull Request Title: Make SearXNG use valkey

Description

SearXNG does not use valkey by default. The settings.yml sets the valkey url to false. This PR takes it into use.
It also replaces strings for 'redis' with 'valkey', since this stack is using valkey, not redis

Related Issues

• None

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring

How Has This Been Tested?

I launched this stack using docker compose up locally and verified valkey is being used by:

• observing no connection errors from app-searxng container
• logging into app-searxng and checking SEARXNG_VALKEY_URL is set in env

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix or feature works
• I have updated necessary documentation (e.g. frontpage README.md)
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

None

Additional Notes

None