Enable duo_workflow_in_ci feature flag by default

What does this MR do and why?

This MR enables the duo_workflow_in_ci feature flag by default for the 18.4 release as requested in https://gitlab.com/gitlab-org/gitlab/-/work_items/560928.

With this change, the availability of remote flows will be with cascading settings through Admin, group and project.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/560928

Author's checklist

• Feature flag definition updated to default_enabled: true
• Commit message references the work item

Reviewer's checklist

• Review the feature flag change
• Confirm this aligns with the 18.4 release timeline