Rubocop: allow methods up to 20 lines long (not counting comments)

Our longest method is currently 16 lines long. Let's give ourselves
a little bit of margin on top of that, while ensuring we don't
regress to much.

The default setting (based on https://rubystyle.guide/#short-methods) is 10
lines, which seems unrealistic and not worth the effort, given the amount of
programming skills we're putting into this script.