Update dependency versions

[florianm]

What are the core team's thoughts on updating dependencies?

• Ruby > 2.7.2
• Gemfile> bundle update
• CircleCI: Ruby 2.7.2 image
  (what did I miss?)

The currently used Ruby version (2.3.3) is now at end of life and doesn't come up with rbenv install -l (still available in rbenv install --list-all).