fix: Correct Helm chart label in Grafana dashboard ConfigMap

Updates the `helm.sh/chart` label in the Grafana dashboard ConfigMap
to use `{{ .Chart.Name }}-{{ .Chart.Version | replace "+" "_" }}`.
This resolves a Helm linting error caused by an incorrect template reference.

The previous commit added the Grafana dashboard as a ConfigMap:
feat: Add Grafana dashboard as ConfigMap

Adds the Grafana dashboard for iperf3-monitor as a ConfigMap to the Helm chart.

The dashboard is sourced from the project's README and stored in
`charts/iperf3-monitor/grafana/iperf3-dashboard.json`.

A new template `charts/iperf3-monitor/templates/grafana-dashboard-configmap.yaml`
creates the ConfigMap, loading the dashboard JSON and labeling it with
`grafana_dashboard: "1"` to enable auto-discovery by Grafana.

charts/iperf3-monitor/templates/grafana-dashboard-configmap.yaml

@@ -5,7 +5,7 @@ metadata:
   labels:
     grafana_dashboard: "1"
     app.kubernetes.io/name: {{ include "iperf3-monitor.name" . }}
-    helm.sh/chart: {{ include "iperf3-monitor.chart" . }}
+    helm.sh/chart: {{ .Chart.Name }}-{{ .Chart.Version | replace "+" "_" }}
     app.kubernetes.io/instance: {{ .Release.Name }}
     app.kubernetes.io/managed-by: {{ .Release.Service }}
 data: