Support Duo Code Review automatic review in customer trials

Problem

Currently, the automatic review is not displayed within Merge Request settings when the namespace is using a $0 subscription, which is the standard mechanism to support a customer trial.

Example settings UI with this problem:

Steps to reproduce

1. Create or update a namespace to use a $0 subscription
2. Ensure the admin user has a Duo seat assigned
3. Navigate to a project where Duo is enabled and access Merge Request settings
4. Observe that Enable automatic reviews by GitLab Duo. option is not available.
   1. Expected: Enable automatic reviews by GitLab Duo. option is available and can be selected.