meta: Re-organize changelog to add v8 page (#17327)

This PR:

- Creates a new individual page for `8.x` changelog entry so that we
reduce the size of the main `CHANGELOG.md`
- Updates the `v7` and `v8` changelogs to be updated to their latest
releases
- Updates the main changelog entry to add v9 releases that were not
tracked before

Author: AbhiPrasad