Update Ruby from 2.7 to 3.0 to use on build/test

Why is this change being made?

To keep the development environment for www-gitlab-com to the latest. Similar initiative is at gitlab-org&5149 (closed).

Author Checklist

• Got !75872 (merged) merged
• Got !78849 (merged) merged
• Got !79022 (merged) merged
• Got !79284 (merged) merged
• Got !79480 (merged) merged
• Got !79484 (merged) merged
• Got !79491 (merged) merged
• Got middleman/middleman#2460 merged (optional)
• Got middleman/middleman#2462 merged (optional)
• 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)

Closes #10917 (closed)