[8.4][DOCS] Adds ML-related release highlights

[szabosteve]

Overview

This PR adds the highlight property with title and some description to the YAML files in the changelog that contain ML-related changes that need to be highlighted.

It also adds the item about "Early stopping for data frame analytics" to the YAML file of the "aggregations in datafeeds are GA". As the early stopping functionality has been added to the stack via a PR in the ML-CPP repo, there is no corresponding YAML file in the elasticsearch repo. This situation and the nature of the highlight generation process necessitate this less elegant way to add the highlight item to the "What's new" section.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)