Bump Ruby dependency to 3.x (or at least 2.7)

[dduportal]

As per https://www.ruby-lang.org/en/downloads/ , Ruby 2.6 is end of line.

We should update this project to run on ruby 2.7, at least (with bundler from 1.17 to 2.x!), or better: to the latest stable 3.x.

#2235 showed us that Ruby 3.x does not work out of the box (or maybe it's bundler 2.x).

The ci.jenkins.io is, for now, defined in the following Docker image: https://github.com/jenkins-infra/docker-inbound-agents/blob/main/ruby/Dockerfile which is quite outdated (Ruby 2.6, Debian stretch).

[dduportal]

As mentioned in jenkins-infra/packer-images#269 (comment): once we'll bump ruby minor (or major) version, we'll have to add an updatecli manifest to track ruby update (mandatory).

[timja]

Ruby 3.x would be great. :)