Describe the Bug:
Alerts are not getting closed in goalert even after resolving in Grafana8.

Steps to Reproduce:

1. Create an alert rule in Grafana (8.1.2) and corresponding notification policy with contact point as goalert incoming webhook.
2. When alert rule goes to alerting state, we get alert in goalert.
3. When the alert get resolved in Grafana, alert is not getting closed in goalert.

Expected Behavior:
Alerts should be closed after it get resolved in Grafana (8.1.2)

Observed Behavior:
Alerts are not getting closed in goalert even after resolving in Grafana8.

Application Version:
Grafana - 8.1.2
Goalert- v0.29.0