docs: Replace ruby:2.1/2.2 with ruby:2.6 or 2.7 (!1748) · Merge requests · GitLab.org / gitlab-runner

What does this MR do?

Replaces ruby:2.1/2.2 in documentation with ruby:2.6/2.7 as Ruby 2.3 or earlier (ruby:2.1, ruby:2.2) have been deprecated in 2019 or earlier.

The changes in Ruby version (2.1 -> 2.6) as the example in the prompt: !1391 (merged)
Ruby EOL: https://www.ruby-lang.org/en/downloads/branches/