[EDU-2056] Add webhook concurrency limit

[m-hulbert]

Description

This PR adds a missing limit for webhook concurrency to the limits page.

Checklist

• [x] Commits have been rebased.
• [x] Linting has been run against the changed file(s).
• [x] The PR adheres to the writing style guide and contribution guide.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment
• [ ] Commit unit tests in branch edu-2056-add-webhook-conc

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