Set preview_path correctly in spec/rails_app (#203)

`preview_path` is deprecated since Rails 7.1, we need to use `preview_paths`.

See: https://blog.saeloun.com/2024/10/12/rails-7-1-supports-multiple-preview-paths-for-mailers/

Author: adamsosterics

spec/rails_app/config/environments/development.rb

@@ -45,7 +45,11 @@
   config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }
 
   # For notification email preview
-  config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
+  if Gem::Version.new("7.1.0") <= Rails.gem_version
+    config.action_mailer.preview_paths << "#{Rails.root}/lib/mailer_previews"
+  else
+    config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
+  end
 
   # Specifies delivery job for mail
   if Rails::VERSION::MAJOR >= 6