Change PagerDuty webhook URL to be scoped URL

What does this MR do?

Closes #230459 (closed)

Changes PagerDuty webhook URL to be /namespace/project/-/incidents/integrations/pagerduty

Since the URL isn't used by anyone yet this change is safe.

Feature flag removal didn't reach production yet !37193 (merged) and the feature was enabled only for https://gitlab.com/gitlab-org/monitor/tanuki-inc project.

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Operations > Incidents

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