Fix Namespace#all_projects performance

Changelog: performance

app/models/namespace.rb

@@ -280,7 +280,7 @@ class Namespace < ApplicationRecord
   # that belongs to this namespace
   def all_projects
     if Feature.enabled?(:recursive_approach_for_all_projects, default_enabled: :yaml)
-      namespace = user? ? self : self_and_descendants
+      namespace = user? ? self : self_and_descendant_ids
       Project.where(namespace: namespace)
     else
       Project.inside_path(full_path)

spec/models/namespace_spec.rb

@@ -1097,6 +1097,14 @@ RSpec.describe Namespace do
       end
 
       include_examples '#all_projects'
+
+      # Using #self_and_descendant instead of #self_and_descendant_ids can produce
+      # very slow queries.
+      it 'calls self_and_descendant_ids' do
+        namespace = create(:group)
+        expect(namespace).to receive(:self_and_descendant_ids)
+        namespace.all_projects
+      end
     end
 
     context 'with use_traversal_ids feature flag disabled' do