[docs] Use centrally maintained version variables

[colleenmcginnis]

Related to https://github.com/elastic/docs-projects/issues/519

Update the docs to use centrally-managed version variables from docs-builder's version.yml configuration file. The Docs team is responsible for updating these version numbers for each release. These version variables will replace docset-level version-related subs so we don't have to update those values in this repo for every release.

[github-actions]

🤖 GitHub comments

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[github-actions]

🔍 Preview links for changed docs

• docs/reference/docker.md
• docs/reference/first-event.md
• docs/reference/installing-logstash.md
• docs/reference/running-logstash-command-line.md
• docs/reference/working-with-plugins.md

[colleenmcginnis]

@karenzone it looks like there are quite a few places with 9.0.0 is hardcoded into the docs. I'm not sure if we should update those to use version variables, too.

• docs/reference/docker-config.md
• docs/reference/first-event.md
• docs/reference/offline-plugins.md
• docs/reference/running-logstash-windows.md

Note: I did not find any hardcoded instances of 9.0 or 9.x that looked like they should be updated to use version variables.

[karenzone]

LGTM