grafana: Enable Unified Alerting and wire up with alertmanager

Grafana v8.2.0 added support for Prometheus's Alertmanager[1] which can
be used with the Unified Alerting added in v8.0.0[2].

[1] https://github.com/grafana/grafana/releases/tag/v8.2.0
[2] https://grafana.com/blog/2021/06/08/grafana-8.0-unified-grafana-and-prometheus-alerts-live-streaming-new-visualizations-and-more/