chore(deps): update dependency @gitlab/ui to v21

This MR contains the following updates:

Package	Type	Update	Change
@gitlab/ui (source)	dependencies	major	20.20.0 -> 21.0.0

MR created with the help of gitlab-org/frontend/renovate-gitlab-bot

---

Release Notes

gitlab-org/gitlab-ui

v21.0.0

Compare Source

Bug Fixes

• dropdowns: Rename section header, remove deprecated exports (771bd3a)

BREAKING CHANGES

• dropdowns: Aligns naming conventions of header and section headers with Figma and removes deprecated exports of GlNewDropdown

---

Renovate configuration

📅 Schedule: At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻️ Rebasing: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this MR and you won't be reminded about this update again.

---

• If you want to rebase/retry this MR, check this box

---

This MR has been generated by Renovate Bot.