Add applies_to frontmatter

[lcawl]

This PR is intended to show the type of applies_to frontmatter that should be included in security reference content (in particular, so that it can be considered in updates to the automated content).

I noticed that there seemed to be at least one deprecated rule, so I updated the frontmatter there appropriately:

---
applies_to:
  stack: deprecated 8.14.0
  serverless:
    security: deprecated all
---

... which is currently displayed as follows:

I've used all for now but going forward per https://docs-v3-preview.elastic.dev/elastic/docs-builder/tree/main/versions/#versions-and-lifecycle-states the appropriate version labels should be used when a feature is added. For example, if there's a new rule added in 9.0.0 or later, that V.R.M should be used in the frontmatter.

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[lcawl]

Closing since the files are now moved to a new repo and frontmatter will need to be automated.

[mergify]

This pull request is now in conflicts. Could you fix it @lcawl? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b frontmatter upstream/frontmatter
git merge upstream/main
git push upstream frontmatter