[Logs Overview] Refine change types

[weltenwort]

:notebook: Summary

The change types currently shown in the log categories data grid are more or less passed through from the change_point aggregation. These types are sometimes hard to interpret as a human or an LLM. Additionally, they are sensitive to small variations in the distribution of the histogram when the document count is low, which leads to confusing flapping between types like spike and step.

• :link: related to: https://github.com/elastic/kibana/issues/190848

:heavy_check_mark: Acceptance criteria

• The changes detected by the Elasticsearch aggregation are post-processed into more useful change types.
• The new change types are more stable with low document counts.

[elasticmachine]

Pinging @elastic/obs-ux-logs-team (Team:obs-ux-logs)