Fix a wrong `X-Gitlab-Event` header when testing webhooks

What does this MR do?

This fixes a wrong X-Gitlab-Event header when testing a webhook: https://requestb.in/1behb381?inspect#1h3nq1

Are there points in the code the reviewer needs to double check?

Not that I can think of.

Why was this MR needed?

Because the "Testing webhook" feature broke inadvertently.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37288 (closed)