Update Orbit default group help text

What does this MR do and why?

Updates the help text of the Default group setting under Preferences > GitLab Orbit from "Usage from MCP and API calls to Orbit is assigned to this group." to "Usage from Orbit is assigned to this group."

The old text is outdated: all Orbit flows are billed to this group, not only MCP and API calls.

References

None.

Screenshots or screen recordings

Before	After

How to set up and validate locally

The Default group setting is hidden unless the user has two or more eligible billing groups. With a single eligible group, that group is used automatically and there is nothing to choose. A group is eligible when the user is Reporter+ in it, Orbit is turned on for it, and it is a top-level group on a Premium or Ultimate plan.

1. Start GDK with SaaS simulation (GITLAB_SIMULATE_SAAS=1, already set in the GDK env.runit).

2. In a Rails console, enable the flag and create two eligible groups for your user:

   user = User.find_by_username('root')
   Feature.enable(:knowledge_graph_billing, user)
   
   groups = 2.times.map do |i|
     group = Group.create!(name: "orbit-billing-#{i}", path: "orbit-billing-#{i}-#{SecureRandom.hex(3)}", organization: user.organization)
     group.add_owner(user)
     (group.gitlab_subscription || GitlabSubscription.create!(namespace: group)).update!(hosted_plan: Plan.find_by!(name: 'ultimate'))
     group
   end

3. Visit http://127.0.0.1:3000/-/profile/preferences. With no group turned on for Orbit, the Default group setting under GitLab Orbit is not shown for these groups.

4. Turn Orbit on for the first group only and reload the page (the user's group list is cached for up to 5 minutes, so wait or clear the cache with Rails.cache.clear). One eligible group: the setting is still hidden.

   Analytics::KnowledgeGraph::EnabledNamespace.create!(root_namespace_id: groups[0].id)

5. Turn Orbit on for the second group and reload. Two eligible groups: the Default group setting appears under GitLab Orbit and reads "Usage from Orbit is assigned to this group."

   Analytics::KnowledgeGraph::EnabledNamespace.create!(root_namespace_id: groups[1].id)

6. If your user already belongs to other Orbit-enabled Premium or Ultimate groups, the setting is already visible. Use a fresh user to see the hidden state.

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