[Elastic Agent] Logs should only contain ecs.version once

[ruflin]

Currently the logs coming from the Agent contain the ecs version twice:

{
   "ecs.version": 1.2.3,
   "ecs": {
      "version": 1.2.3
   }
}

This works with ECS as it combines the two and treats it as array. In any case, we should send it only once.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)