Add Ruby 3 build, default to 2.7
This MR addresses the following problems:
- Update default Ruby from 2.5 to 2.7
- Add Ruby 3 build
- Require
json
>= 2.5 for Ruby 3 compatibility (already used by the main app as of gitlab-org/gitlab!70396 (merged)) - Generate
Gemfile
dependencies from gemspec
Edited by Matthias Käppler