Update ECS version to 9.0

[mjwolf]

To prepare for the upcoming release of ECS 9.0, update the ECS version in documentation to 9.0

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Do not merge yet, ECS 9.0 isn't released yet

[github-actions]

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[colleenmcginnis]

@mjwolf I don't think you need this PR. The build is failing because there are no AsciiDoc files on the 9.0 branch — that branch contains Markdown files instead.

This repo is for building the old AsciiDoc docs, and we're not planning to add any 9.0+ docs to the AsciiDoc-powered site (https://www.elastic.co/guide/index.html). Content for the latest version is in Markdown files, the pages are built with elastic/docs-builder, and they are published to https://www.elastic.co/docs (instead of /guide/).