deps: Ruby 2.7.4

[pinheadmz]

2.7.4 is the latest ruby version in debian stable according to https://packages.debian.org/stable/ruby/ruby

upgraded deps but kept htmlproofer on major version 3 because of: https://github.com/gjtorikian/html-proofer/issues/722

Opening as draft now to test against CI

[fanquake]

Note that bumping this would likely also require changes to server infra.

[fanquake]

Actually, dupe of #826,

[pinheadmz]

🤦