8.6: Unique input IDs are now mandatory. Add an obvious changelog entry and adjust examples accordingly.

[cmacknz]

We need to do a few things to account for this in 8.6:

• Make it really obvious what has changed and how to correct it in the agent release notes.
• Adjust all examples that do not use unique IDs. This includes standalone agent documentation and our example K8s manifests:
  • https://github.com/elastic/observability-docs/tree/main/docs/en/ingest-management/elastic-agent
  • https://github.com/elastic/elastic-agent/blob/main/deploy/kubernetes/elastic-agent-standalone/elastic-agent-standalone-daemonset-configmap.yaml
  • Add an input ID to the default agent policy.
• Adjust the error message to make the correction to the agent policy as obvious as possible. The current error message is:

{"log.level":"error","@timestamp":"2022-12-22T14:26:37.153Z","log.origin":{"file.name":"coordinator/coordinator.go","file.line":580},"message":"failed to render components: invalid 'inputs.3.id', has a duplicate id \"kubernetes/metrics\" (id is required to be unique)","ecs.version":"1.6.0"}

[cmacknz]

This will be closed by elastic/observability-docs#2479 to update the agent docs.