Disband GitLab UI working group

Why is this change being made?

This MR officially disbands the GitLab UI working group. Since creation, the group has made huge progress in the building of reusable components, including documentation for their use. With the creation of a Foundations team, we have decided that it is time to disband the working group and invite members to the Foundations open office hours for continued participation outside of a dedicated working group.

Author Checklist

• Correct MR template applied (e.g blog post)
• 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 your manager.
  • 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.

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