Add support for Rails 7 alpha/beta versions

[akodkod]

Some people (like me) need to use alpha version of Ruby to run it using Ruby 3.1. Please, check official thread: rails/rails#43998

[rossta]

My understanding is that Ruby 3.1 support was added to Rails 7.0.1 which montrose 0.13.0 supports officially

https://rubyonrails.org/2022/1/6/Rails-7-0-1-has-been-released