Trigger webhooks when all discussions have been resolved

What does this MR do?

Triggers :merge_request_hooks when all discussion have been resolved.
Partially-fixes: #26234 (closed)

How to setup and validate locally

1. Start GitLab instance gdk start
2. Add a webhook address to the project > Settings > Webhooks.
3. Enable "All discussion must be resolved" under project > Settings > Merge Request.
4. Open a discussion in an MR.
5. Resolve the discussion and check the webhook address for a webhook event.

Conformity

• [y] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
  • I'm not sure if we should mention on the :project/-/hooks page about discussion also triggering webhooks.
• [y] This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)