Remove 8.x release notes from main #6041
Description
main = 9.0
Related to: https://github.com/elastic/docs-projects/issues/257
Creating this issue as a reminder to clear release notes, breaking changes, and what's new highlights from the 8.x series.
We need to do this before the 9.0.0-beta1 scheduled for feature freeze on Jan 15, 2025.