Update html-proofer to 3.12.2

Why is this change being made?

Updates html-proofer Gem from 3.10.2 to 3.12.2 to remove unnecessary activesupport (< 6.0!) dependency via gjtorikian/html-proofer#534 (included in html-proofer 3.12.0).

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

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.
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  • 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)