Use active version of Redis for an example

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

changelogs/unreleased/docs-redis-6-on-ci-services.yml

+---
+title: Use active version of Redis for an example
+merge_request: 39404
+author: Takuya Noguchi
+type: other

doc/ci/services/redis.md

@@ -34,7 +34,7 @@ And that's it. Redis will now be available to be used within your testing
 framework.
 
 You can also use any other Docker image available on [Docker Hub](https://hub.docker.com/_/redis).
-For example, to use Redis 2.8 the service becomes `redis:2.8`.
+For example, to use Redis 6.0 the service becomes `redis:6.0`.
 
 ## Use Redis with the Shell executor