[neutron] Change alert severity for API rate-limited projects

The alert channel is getting too many notifications for projects exceeding the neutron API limits. We decide to change the severity from `warning` to `info`. This change implies:
1) no more notifications on the alert channel on "local/per project" rate limit violations: `info` alerts do not get routed to the alerting system 
2) alerts are still accessible on Thanos via GreenHouse

openstack/neutron/alerts/openstack/api.alerts

@@ -42,7 +42,7 @@ groups:
             sum by (target_type_uri, action, target_project_id) (rate(openstack_watcher_api_requests_total{service="network", container="statsd"}[1m])) > 0.1
     for: 5m
     labels:
-      severity: warning
+      severity: info
       support_group: network-api
       tier: os
       service: neutron