fix(chart): make Grafana dashboard datasource portable

[ynsta]

Summary

The bundled Grafana dashboard hard-coded datasource.uid: prometheus, so
it only rendered on clusters whose Prometheus data source happened to be
named prometheus. This patch adds a datasource template variable
(type datasource, query prometheus) and rewrites every panel target
plus the existing instance query to reference ${datasource}.

Chart bump 1.9.0 → 1.9.1. No values changes; upgrade is in-place.

See CHANGELOG.md → Chart / 1.9.1.

Test plan

• helm lint charts/s3proxy
• jq empty charts/s3proxy/dashboards/s3proxy.json
• Render emits chart: s3proxy-1.9.1 and the new template var:

  helm template t charts/s3proxy \
    --set grafanaDashboard.enabled=true \
    --set grafanaDashboard.folder=s3proxy \
    --show-only templates/grafana-dashboard.yaml
  

  Expect a templating.list entry with "name": "datasource",
  "type": "datasource", "query": "prometheus", and every panel
  target carrying "datasource": {"type": "prometheus", "uid": "${datasource}"}.
• Apply against a real Grafana instance whose Prometheus DS is not
  named prometheus; confirm the dashboard renders and the data source
  picker is populated.
• CI green on chart/dashboard-datasource-var.

Release flow

Merge → tag chart-v1.9.1 on main → CI publishes
ghcr.io/intrinsec/s3proxy/charts/s3proxy:1.9.1.

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