Add Elastic Agent 9.3.1 release notes

[elastic-vault-github-plugin-prod]

Adds Elastic Agent 9.3.1 release notes.

Checklist

• @ebeahan review this PR to make sure the correct items are included
• @elastic/ingest-docs review the language
• @ebeahan merge this PR and forwardports on release day
  • Note: In this repo, we publish from the main branch. The changes in this PR will not be available on the docs site until they are added to the main branch.

[github-actions]

✅ Vale Linting Results

No issues found on modified lines!

---

The Vale linter checks documentation changes against the Elastic Docs style guide.

To use Vale locally or report issues, refer to Elastic style guide for Vale.

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
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 release-notes-9.3.1 upstream/release-notes-9.3.1
git merge upstream/9.3
git push upstream release-notes-9.3.1

[karenzone]

Please consider the style suggestions from the linter.
I addressed the conflict, but will re-check in doc build.

[ebeahan]

Please consider the style suggestions from the linter.

@karenzone revised with the suggestions.

[karenzone]

Suggested change

	:::{include} /release-notes/_snippets/9.3.1/breaking-changes.md
	:::
	:::{include} /release-notes/_snippets/9.3.1/breaking-changes.md
	:::
	:::{include} /release-notes/_snippets/9.3.1/breaking-changes.md
	:::

[karenzone]

Suggested change

	:::{include} /release-notes/_snippets/9.3.1/deprecations.md
	:::
	:::{include} /release-notes/_snippets/9.3.1/deprecations.md
	:::
	:::{include} /release-notes/_snippets/9.3.1/deprecations.md
	:::

[karenzone]

Suggested change

	:::{include} /release-notes/_snippets/9.3.1/index.md
	:::
	:::{include} /release-notes/_snippets/9.3.1/index.md
	:::

[karenzone]

Not sure why tooling is spitting our duplicate entries. Sorry I didn't catch this the first time through.