feat(monitoring): Add persistence to alertmanager

This volume should help to keep silenced alarms silenced and already
alerted alerts unalerted, when alertmanager is restarted or alike.

infrastructure/monitoring/release.yaml

@@ -52,7 +52,15 @@ spec:
       alertmanagerSpec:
         replicas: 1
         retention: 120h
-        storage: {}
+        storage:
+          volumeClaimTemplate:
+            spec:
+              storageClassName: longhorn
+              accessModes: ["ReadWriteOnce"]
+              resources:
+                requests:
+                  storage: 1Gi
+
         resources:
          requests:
            memory: 400Mi