route SLO alerts to pagerduty

We intended to route general service SLO alerts to #production and pagerduty a few weeks ago but missed a detail to make it work. This should fix it.

Now all alerts in https://gitlab.com/gitlab-com/runbooks/blob/master/rules/general-service-alerts.yml marked with pagerduty will actually page instead of going to #alerts-general. This will lead to more pages but if we had that a few weeks ago we could have detected some of the severe incidents much earlier (e.g. gitlab-com/gl-infra/production#1013 (closed)).

closes https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7391

/cc @andrewn /cc @craigf @ahanselka for on-call awareness.