fix(dashboard): update Random Sensor dashboard for Grafana 10+ compatibility

The original dashboard was built for Grafana 7.3 and uses panel types and
plugin references that are broken on Grafana 10+:

- `graph` panel type removed in Grafana 10 → replaced with `timeseries`
- `grafana-piechart-panel` (AngularJS) disabled in Grafana 11+ → replaced
  with native `piechart` panel
- `datasource: null` on all panels → replaced with explicit datasource
  reference (type + uid) required by Grafana 8+ schema
- `grafana-kairosdb-datasource` type → updated to `arpnetworking-kairosdb-datasource`
  (React fork compatible with Grafana 11+)
- Removed AngularJS artifacts ($$hashKey fields)
- Removed deprecated histogram panel (incompatible data format)
- Updated schemaVersion from 26 to 39

Author: Simone Maiorani