Trigger webhook for more than one unresolved discussion in merge requests

Problem to Solve

In #26234 (closed) a webhook was added that triggered when all discussions were resolved. A corresponding trigger for discussions be unresolved still needs to be added.

Proposal

Instead of triggering a webhook for each unresolved thread event, we should only trigger the webhook one time if the number of unresolved threads is going from 0 to more than 1.