[Docs] Reset release files for 9.0

[florent-leborgne]

This PR resets the release notes, upgrade notes, and what's new for 9.0.
It also cleans up a few references/files that were focusing on migration to 8.0

Some more PRs will happen to prepare the rest of the docs for v9

Closes: https://github.com/elastic/platform-docs-team/issues/564

[elasticmachine]

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

[github-actions]

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[karenzone]

WRT Breaking changes. Not all users are on the latest version, and we should support those users' upgrade paths, too. Should we keep the 8.0 breaking changes toward that goal?

Logstash has fewer breaking changes than ES or Kibana, but here's how we handle breaking changes: https://www.elastic.co/guide/en/logstash/current/breaking-changes.html#_breaking_changes_in_earlier_releases.

Breaking changes might be a candidate for the next round of IA projects.

[florent-leborgne]

@karenzone good point. To upgrade to 9.0, users will have to upgrade to the last 8.x first. So in the upgrade docs we will call this out and link to the 8.x breaking change summary as well. That should help us keep only 9.x content in those files for that branch.

[kilfoyle]

LGTM! 🦘