Draft: Update html-proofer to 3.18.8

Why is this change being made?

Updates html-proofer Gem from 3.12.2 to 3.18.8 to keep dependency up-to-date and to change indirectly parser from nokogiri to nokogumbo https://github.com/gjtorikian/html-proofer/pull/362 done in html-proofer 3.15.0.

The gem was pinned in !42571 without any explanation, so we can safely update it in order to achieve #10917.

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)