Fix a build error

Follow rubocop/rubocop#9197.

This PR fixes the following build error.

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

Failures:

  1) RuboCop Performance Project default configuration file has a nicely
  formatted description for all cops
     Failure/Error: subject(:config) {
  RuboCop::ConfigLoader.load_file('config/default.yml') }

     RuboCop::ValidationError:
       `Performance` cops have been extracted to the
       `rubocop-performance` 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:16:in `block (4 levels) in
       <top (required)>'
     # ./spec/project_spec.rb:15:in `each'
     # ./spec/project_spec.rb:15: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-performance') # rubocop:disable RSpec/AnyInstance
+    end
+
     let(:registry) { RuboCop::Cop::Registry.global }
     let(:cop_names) do
       registry.with_department(:Performance).cops.map(&:cop_name)