Update Ruby from 2.6 to 2.7 to use on build/test

Why is this change being made?

We need continuously keep updating the build/test environments. This does not include updating Bundler from 1.17.x to 2.1.4 (that is the scope of #10753 (closed)).

cf.

gitlab-org/gitlab-build-images!356 (merged) recently prepared the Ruby 2.7 image.

What

Upgrades middleman gem from 4.3.11 (620ba3d) to 'ffde2fe': diff in the upstream: https://github.com/middleman/middleman/compare/620ba3d...ffde2fe

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)
• There is no deprecation warning on handbook-build-and-review-deploy 1/2 and handbook-build-and-review-deploy 2/2 jobs: https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/1029557410 https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/1029557411