CI against Rails 7.1

[y-yagi]

This PR adds Rails 7.1 to the CI matrix to ensure the gem works with Rails 7.1.

This PR includes two commits to pass the test.

• Ignore the directory that keeps factory files under the lib
  Some tests put factory files under the lib. But since Rails 7.1, Rails loads lib directory by default in a new application.
  https://guides.rubyonrails.org/7_1_release_notes.html#introducing-config-autoload-lib-and-config-autoload-lib-once-for-enhanced-autoloading
  But factory files don't follow the naming rule of Zeitwerk. So Zeitwerk raises Zeitwerk::NameError. To avoid the error, this
  changed to ignore the directory that puts factory files.

• Remove the route that is defined by default
  Some test code loads the route file twice. This wasn't a problem before Rails 7.0 because we didn't have a route.
  But, Rails 7.1 has one route by default. So the test application raises the following error during the load.

  Invalid route name, already in use: 'rails_health_check'  (ArgumentError)
  You may have defined two routes with the same name using the `:as` option, or you may be overriding a route already 
  defined by a resource with the same naming. For the latter, you can restrict the routes created with `resources` as explained 
  here:
  https://guides.rubyonrails.org/routing.html#restricting-the-routes-created
  

  The routing isn't a matter of this gem, so just ignore that for running the test.

[neilvcarvalho]

This PR's commits were cherry-picked to #440, and released on https://github.com/thoughtbot/factory_bot_rails/releases/tag/v6.4.2

Thank you for working on this!