Update backend-push-notifications image to version 15

Linked build commit: e267647fc9

backend-push-notifications/deployment.yaml

@@ -50,7 +50,7 @@ spec:
               mountPath: /etc/secrets
       containers:
         - name: backend-push-notifications
-          image: ghcr.io/gansejunge/app-notifications-backend-push-notifications:14
+          image: ghcr.io/gansejunge/app-notifications-backend-push-notifications:15
           ports:
             - containerPort: 9000
               name: metrics