fix: set priority class name for metallb (#1046)

**What problem does this PR solve?**:
Set Priority class name for metallb controller and speaker pods
https://github.com/metallb/metallb/tree/main/charts/metallb

**Which issue(s) this PR fixes**:
Fixes #
https://jira.nutanix.com/browse/NCN-105635
**How Has This Been Tested?**:
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Author: supershal

charts/cluster-api-runtime-extensions-nutanix/addons/serviceloadbalancer/metallb/values-template.yaml

@@ -1,4 +1,5 @@
 controller:
+  priorityClassName: system-cluster-critical
   tolerations:
     - key: node-role.kubernetes.io/control-plane
       effect: NoSchedule
@@ -9,6 +10,7 @@ controller:
       operator: Exists
       tolerationSeconds: 300
 speaker:
+  priorityClassName: system-cluster-critical
   tolerations:
     - key: node-role.kubernetes.io/control-plane
       effect: NoSchedule