Bump CI Redis dependency from 4.0 to 6.0

What does this MR do?

We're looking into upgrading from Redis 5 to Redis 6 for GitLab.com (gitlab-com/gl-infra&395 (closed)).

We have an omnibus change here: omnibus-gitlab!4930 (merged).

Before merging that, we want to make sure the Rails CI runs successfully against the new Redis version.

We also noticed that we'd still been running against 4.0 here. Not sure if that was by design, and whether we need to keep running against both 5.0 and 6.0 to ensure we don't rely on features from 6.0.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

refs https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/12419