Exclude the latest alert groups from escalation finished check (#2913)

# What this PR does
Exclude the latest alert groups from escalation finished check to give
them time for building escalation snapshot

## Which issue(s) this PR fixes
https://github.com/grafana/oncall-private/issues/2028

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

engine/apps/alerts/tasks/check_escalation_finished.py

@@ -114,7 +114,7 @@ def check_escalation_finished_task() -> None:
     """
     from apps.alerts.models import AlertGroup
 
-    now = timezone.now()
+    now = timezone.now() - datetime.timedelta(minutes=5)
     two_days_ago = now - datetime.timedelta(days=2)
 
     alert_groups = AlertGroup.objects.using(get_random_readonly_database_key_if_present_otherwise_default()).filter(