Update Node.js to v14.21

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
node	stage	minor	14.18-slim -> 14.21-slim
node		minor	14.18-slim -> 14.21-slim
node	docker	minor	14.18-buster-slim -> 14.21-buster-slim

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Enabled.

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

🔕 Ignore: Close this PR and you won't be reminded about these updates again.

---

• [ ] If you want to rebase/retry this PR, check this box

---

This PR has been generated by Mend Renovate. View repository job log here.