Pin concurrent-ruby on old Rails to supported version

Author: JonRowe

Gemfile-rails-dependencies

@@ -49,6 +49,8 @@ when nil, false, ""
 else
   version_number = version.split(' ').last
 
+  gem 'concurrent-ruby', '1.3.4' if version_number < '7.1'
+
   gem "rails", version
   gem 'activerecord-jdbcsqlite3-adapter', platforms: [:jruby]