Fix a build error

Follow rubocop/rubocop#9197.

This PR fixes the following build error.

```console
% cd path/to/rubocop-rails
% bundle exec rspec spec/project_spec.rb
(snip)

Failures:

  1) RuboCop Rails Project default configuration file has a
  SupportedStyles for all EnforcedStyle and EnforcedStyle is valid
     Failure/Error: subject(:config) {
  RuboCop::ConfigLoader.load_file('config/default.yml') }

     RuboCop::ValidationError:
       `Rails` cops have been extracted to the `rubocop-rails` gem.
       (obsolete configuration found in config/default.yml, please
       update it)
     # ./spec/project_spec.rb:5:in `block (3 levels) in
       <top (required)>'
     # ./spec/project_spec.rb:53:in `block (4 levels) in
       <top (required)>'
     # ./spec/project_spec.rb:52:in `each'
     # ./spec/project_spec.rb:52:in `block (3 levels) in <top (required)>'
```

spec/project_spec.rb

@@ -4,6 +4,10 @@
   describe 'default configuration file' do
     subject(:config) { RuboCop::ConfigLoader.load_file('config/default.yml') }
 
+    before do
+      allow_any_instance_of(RuboCop::Config).to receive(:loaded_features).and_return('rubocop-rails') # rubocop:disable RSpec/AnyInstance
+    end
+
     let(:registry) { RuboCop::Cop::Registry.global }
     let(:cop_names) do
       registry.with_department(:Rails).cops.map(&:cop_name)