Update Node.js to v15 - autoclosed

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
circleci/node	docker	major	12-browsers -> 15-browsers

:information_source: Find our documentation at https://github.com/Financial-Times/next/wiki/Renovate.

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Configuration

📅 Schedule: "after 10pm every weekday,before 5am every weekday,every weekend" in timezone Europe/London.

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

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

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

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

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