Merge PR #24: dashboard Grafana 10+/11+ compatibility (Simone Maiorani)

Updates dashboards/Examon Test - Random Sensor.json for Grafana 10+/11+:
- graph -> timeseries panel
- grafana-piechart-panel (AngularJS) -> native piechart
- explicit datasource references (type + uid)
- grafana-kairosdb-datasource -> arpnetworking-kairosdb-datasource
  (paired with Issue #25 fix in subsequent commits)
- removed deprecated histogram and AngularJS artifacts
- schema version 26 -> 39

The pre-merge legacy dashboard is preserved at
dashboards/legacy/Examon Test - Random Sensor.json for users of the
frozen docker-compose v0.4.0 stack.

Co-authored-by: Simone Maiorani <simone.maiorani@e4company.com>

Author: fbeneventi