Exclude guests being billable for ultimate trials

What does this MR do and why?

Guest users are not considered billable / do not take up a seat for our Ultimate plan customers.

This MR applies that same logic to customers who are on the Ultimate Trial plan

How to set up and validate locally

1. Modify Gitlab.com? to return true here, this may require an instance restart to take effect
2. Create a new group, e.g. ultimate-trial-group
3. Apply an ultimate trial for that group (via http://gdk.test:3000/-/trials/new)
4. Add a guest user and a billable user (e.g. maintainer) to the group (via http://gdk.test:3000/groups/ultimate-trial-group/-/group_members) or a project within the hierarchy
5. Confirm the guest user is not shown as billable (isn't present) in the seat usage screen (http://gdk.test:3000/groups/ultimate-trial-group/-/usage_quotas#seats-quota-tab)
6. Checkout master and confirm that the guest user is listed and shown as billable (incorrectly)

Refs #352402 (closed)

MR acceptance checklist

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