Changed unit_unavailable interval for prometheus

As stated in issue canonical/bundle-kubeflow#564
the duation for alerts for argo is set to 0m, which is too low for prod
environments. We need to change to at least 5m to prevent the flapping behavior.

Partial-Bug: canonical/bundle-kubeflow#564

charms/jupyter-controller/src/prometheus_alert_rules/unit_unavailable.rule

@@ -1,6 +1,6 @@
 alert: JupyterControllerUnitIsUnavailable
 expr: up < 1
-for: 0m
+for: 5m
 labels:
   severity: critical
 annotations: