It turned out, that pushing all warning to GitLab resulted in being too
noisy. There were more than 30 issues opened in no time, often
duplicates, due to how GitLab maps or rather doesn't map alerts in the
free version.

This reverts commit de6e02d9.

This patch enables the delivery of alerts with the severity warning to
GitLab, which allows to handle the issues without causing alert fatique.

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