Add the DeviceHealthTruncation cron back (#2098)

This looks to have been removed by mistake

Author: joshk

config/config.exs

@@ -76,6 +76,7 @@ config :nerves_hub, Oban,
        {"*/1 * * * *", NervesHub.Workers.CleanStaleDeviceConnections},
        {"1,16,31,46 * * * *", NervesHub.Workers.DeleteOldDeviceConnections},
        {"*/5 * * * *", NervesHub.Workers.ExpireInflightUpdates},
+       {"*/15 * * * *", NervesHub.Workers.DeviceHealthTruncation},
        {"*/15 * * * *", NervesHub.Workers.MonitorDeploymentOrchestrators}
      ]}
   ]