Operator dashboard widget targeting incorrect podname

Queries that check the operator pod are not using the correct pod name:

https://github.com/cloudnative-pg/grafana-dashboards/blob/main/charts/cluster/grafana-dashboard.json#L2238

`sum(kube_pod_status_ready{namespace=\"$operatorNamespace\", pod=~\"cloudnative-pg.+|cnpg-controller-manager.+\", condition=\"true\"})`

The pod launched when using the operator helm chart:

```
cloudnativepg-cloudnative-pg-b6f5896b7-5jdcc 1/1 Running 0 176m
```

`cloudnative-pg != cloudnativepg`

Happy to create a PR to fix.

Open charts/cluster/grafana-dashboard.json and start at line 2238, then compare the pod selector with the pod name produced by the operator Helm chart. Update the targeting so the dashboard query matches the launched operator pod, and verify that the dashboard JSON remains valid.

Written by the indexing model from the issue text.