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
Has been reviewed by Backend
Conform by the style guides
Squashed related commits together
What are the relevant issue numbers?
Closes #37288 (closed)