Avoid broken versions of factory_bot_rails

See thoughtbot/factory_bot_rails#431.

Gemfile

@@ -38,7 +38,8 @@ gem 'rspec_junit_formatter', require: false
 gem 'yard', require: false
 
 # Ensure the requirement is also updated in core/lib/spree/testing_support/factory_bot.rb
-gem 'factory_bot_rails', '>= 4.8', require: false
+# TODO: remove the "< 6.3" requirement once https://github.com/thoughtbot/factory_bot_rails/issues/431 is fixed.
+gem 'factory_bot_rails', ['>= 4.8', '< 6.3'], require: false
 
 group :backend do
   gem 'capybara', '~> 3.13', require: false