Audit event "Streams" tab should not be in subgroups UI

Summary

Streaming audit event destinations are currently visible in the UI of subgroups. They should only be visible and configurable at the top-level group.

Steps to reproduce

Example Project

What is the current bug behavior?

The "Streams" tab is currently visible when viewing a subgroup's audit event page.

What is the expected correct behavior?

The "Streams" tab should not be visible when viewing a subgroup's audit event page.

Relevant logs and/or screenshots

Implementation plan

frontend - 2️⃣

1. Update https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/views/groups/audit_events/index.html.haml#L12 to check for if the group is a root-level group by using @group.subgroup?
2. Update specs to validate the stream tab doesn't appear on subgroups
3. Update documentation to make it clear its only available on root-groups:

-Event streaming allows owners of top-level groups to set an HTTP endpoint to receive all audit events about the group, and its subgroups and projects as structured JSON.
+Event streaming is a top-level group feature that allows users with the Owner role to set an HTTP endpoint to receive all audit events about the group, and its subgroups and projects as structured JSON.