Make subgroup_creation_level default to maintainer at SQL level

- Migration updates existing groups to "owner", then sets default to
  "maintainer" so that new groups will default to that
- Update spec examples

spec/factories/groups.rb

@@ -5,7 +5,6 @@ FactoryBot.define do
     type 'Group'
     owner nil
     project_creation_level ::Gitlab::Access::MAINTAINER_PROJECT_ACCESS
-    subgroup_creation_level ::Gitlab::Access::OWNER_SUBGROUP_ACCESS
 
     after(:create) do |group|
       if group.owner

spec/models/group_spec.rb

@@ -997,9 +997,8 @@ describe Group do
 
   describe 'subgroup_creation_level' do
     it 'defaults to maintainers' do
-      group = create (:group)
-
-      expect(group.subgroup_creation_level).to eq(Gitlab::Access::MAINTAINER_SUBGROUP_ACCESS)
+      expect(group.subgroup_creation_level)
+        .to eq(Gitlab::Access::MAINTAINER_SUBGROUP_ACCESS)
     end
   end
 end

spec/requests/api/groups_spec.rb

@@ -803,10 +803,10 @@ describe API::Groups do
           group2.add_maintainer(user1)
         end
 
-        it 'cannot create subgroups' do
+        it 'can create subgroups' do
           post api("/groups", user1), params: { parent_id: group2.id, name: 'foo', path: 'foo' }
 
-          expect(response).to have_gitlab_http_status(403)
+          expect(response).to have_gitlab_http_status(201)
         end
       end
     end