fix(metallb): Add required service account reference

The upgrade to 0.12.x started to require a serviceaccount for the
podmonitor. This patch adds the kube-prometheus-stack service account to
the config.

infrastructure/metallb/release.yaml

@@ -26,3 +26,5 @@ spec:
         enabled: true
       prometheusRule:
         enabled: true
+      serviceAccount: kube-prometheus-stack-prometheus
+      namespace: monitoring-system