Move feature repositories to Ruby 2.7 and update gems support for 2.6 and above

[AlexB52]

The EOL for 2.6 comes at the end of March.

• We need to update retest gem to support 2.6+
• We need to update the feature repositories to Ruby 2.7. List of repositories:

• [ ] ruby-app
• [ ] ruby-bare
• [ ] git-ruby
• [ ] hanami-app
• [ ] rails-app
• [ ] rspec-rails
• [ ] rspec-ruby

I still wonder if we need this. As long as we support 2.4 then we should be good.

Create an app using ruby 3 could be helpful though.