Update policy for seat_usage_controller#show to match API

Update policy check from `authorize_admin_group` to
`authorize_admin_group_member`, in order to match API -
https://gitlab.com/gitlab-org/gitlab/-/blob/cb26a88c1852289bb19b11a8231610e0c2c72a49/ee/lib/ee/api/members.rb#L69.

https://gitlab.com/gitlab-org/gitlab/-/issues/262877

ee/app/controllers/groups/seat_usage_controller.rb

 # frozen_string_literal: true
 
 class Groups::SeatUsageController < Groups::ApplicationController
-  before_action :authorize_admin_group!
+  before_action :authorize_admin_group_member!
   before_action :verify_namespace_plan_check_enabled
 
   layout "group_settings"

ee/spec/controllers/groups/seat_usage_controller_spec.rb

@@ -36,10 +36,10 @@ RSpec.describe Groups::SeatUsageController do
         group.add_developer(user)
       end
 
-      it 'renders 404 when user is not an owner' do
+      it 'renders 403 when user is not an owner' do
         get_show
 
-        is_expected.to have_gitlab_http_status(:not_found)
+        is_expected.to have_gitlab_http_status(:forbidden)
       end
     end
   end