Update Ruby gems and use Ruby 2.7.6 Docker images

Full regeneration of Gemfile.lock. Incorporates: https://rubygems.org/gems/gitlab_kramdown/versions/0.20.0.

• Because new gitlab_kramdown requires Ruby 2.7.6, update docker images used. This uses newly-combined base/boostrap image: !2768 (merged).
• Because update to 2.7.6 seems to be affected by: https://www.ctrl.blog/entry/ruby-psych4.html, update call when reading YAML file.