Prevent use of code suggestions at subgroup level

What does this MR do and why?

Disables subgroup setting of code suggestions as it is only observed in the code at the top level group/namespace.

See discussion in https://gitlab.com/gitlab-org/gitlab/-/issues/405126#note_1346731252

Screenshots or screen recordings

n/a - to see how subgroups appear on master branch/normally see !117017 (merged)

How to set up and validate locally

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
2. Enable the ai_assist_ui and ai_assist_flag

   Feature.enable(:ai_assist_ui)
   Feature.enable(:ai_assist_flag)

3. Create new top level group or use an existing.
4. Visit the admin path to edit the plan of the group such as: http://gdk.test:3000/admin/groups/non-owner-notification/edit
5. Update the plan to be Ultimate and Save

6. Create a subgroup on that group that was created.
7. Visit the subgroup's Settings -> General -> Permissions and group features
8. Validate Code Suggestions is no longer seen as compared to master.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #405126