Fix a bug when checking future renewals for a subgroup

Previously, when checking if a future renewal for a subscription was
present, the service would assume that the namespace being passed to it
was the one with the gitlab subscription. This is not always the case,
so now it will check the root ancestor of the given namespace

ee/app/services/gitlab_subscriptions/check_future_renewal_service.rb

@@ -10,11 +10,12 @@
 module GitlabSubscriptions
   class CheckFutureRenewalService
     def initialize(namespace:)
-      @namespace = namespace
+      @namespace = namespace.root_ancestor
     end
 
     def execute
       return false unless Feature.enabled?(:gitlab_subscription_future_renewal, default_enabled: :yaml)
+      return false unless namespace.gitlab_subscription.present?
 
       future_renewal
     end

ee/spec/services/gitlab_subscriptions/check_future_renewal_service_spec.rb

@@ -45,6 +45,23 @@ RSpec.describe GitlabSubscriptions::CheckFutureRenewalService, :use_clean_rails_
       end
     end
 
+    context 'when called with a sub-group' do
+      let(:root_namespace) { create(:group_with_plan) }
+      let(:namespace) { build(:group, parent: root_namespace) }
+
+      it 'uses the root ancestor namespace' do
+        expect(Gitlab::SubscriptionPortal::Client).to receive(:subscription_last_term).with(root_namespace.id).and_return({})
+
+        execute_service
+      end
+    end
+
+    context 'when the namespace has no plan' do
+      let(:namespace) { build(:group) }
+
+      it { is_expected.to be false }
+    end
+
     context 'when the `gitlab_subscription_future_renewal` feature flag is disabled' do
       before do
         stub_feature_flags(gitlab_subscription_future_renewal: false)