Add webhook for changing incident severity

We are in the process of moving GitLab incident management from GitLab issues, to incident types https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13559

Now that we can open incidents through the API this is possible, though we need a way to trigger a webhook when severity is updated. Currently this doesn't count as an "Issue Event"

When Severity is updated here:

What we would like is to have the option of sending a webhook when Severity is updated, this is important because changing the Severity needs to send a notification to our management Slack channel that the status of the incident has changed.

There is another issue related to this which is to add alert types to webhooks #299588