[Ingest Manager] Discuss dataset -> datastream / data_stream change

[skh]

This issue is to discuss what needs to be done if https://github.com/elastic/observability-dev/issues/1028#issuecomment-667909388 Option 2 is chosen for 7.9

• Adjust dataset aggregation in data_stream handler to use datastream fields instead
  • Ensure all frontend code still works after that change
• updating indices during package update used to use dataset before using the datastream api. it does still reference the index template dataset property to compose the stream name.
• ...?

[elasticmachine]

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

[jen-huang]

Closing as the work in #74342 is merged. I couldn't think of any other areas that need changing to support this. Feel free to reopen if there is additional work needed.