[EDU-2056] Add webhook concurrency limit

[m-hulbert]

Description

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

Checklist

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

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✨ Finishing Touches

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[franrob-projects]

I've picked this one up @SimonWoolf @m-hulbert !