Resolve "Wiki page delete does not trigger the webhook"

What does this MR do?

Changes the behaviour of the webhooks associated with Wiki Page events to also be fired upon Wiki Page deletion. Currently, the webhooks for Wiki Page events are only fired upon creating and updating Wiki Pages.

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

1. Please verify the changes to the WikiPage factory. WikiPages aren't models so the persistence of the resource differs.
2. Should the service be called DestroyService or DeleteService?
3. To squash or not to squash?

Why was this MR needed?

A bug was reported that webhooks were only fired for create and update operations upon Wiki Pages.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated - [] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #19302 (closed)