The usage of the trusted-ip config resulted in a security incident that
allowed access to any oauth2-proxy protected endpoint without requiring
authentication.

Thankfully all significant endpoints had been protected by additional
measures such as network restrictions and are therefore not affected.
Only the prometheus and alertmanager endpoints have been exposed to the
public internet, but are not exposing sensitive data beyond metrics.

A check of the relevant logs didn't provide any indication of
compromise.

This patch replaces the third-party integration with the GitLab Native
integration for alertmanager alerts using webhooks.

This should play nicer with Alertmanager and GitLab and also resolve
issues, when the alert is marked as resolved or silenced.

References:
https://docs.gitlab.com/ee/operations/incident_management/integrations.html#add-integration-credentials-to-prometheus-alertmanager