Enable automatic incident creation in incident.io

We decided we should automatically create triage incidents from alerts in #26474 (closed).

We can automatically create incidents based on alerts from alertmanager that have the label pager=pagerduty. Creating incidents for every alert would be extremely noisy and not particularly useful.

We have enabled alertmanager sending alerts to our incident.io sandbox in #26498 (closed) so we can test there. Once we are satisfied, we can migrate the configuration to our incident.io production environment.