Update changelog style guide for new versions

[lcawl]

Summary

Relates to elastic/docs-builder#3956

This PR updates https://www.elastic.co/docs/contribute-docs/content-types/changelogs for the recent changes to the target and versions fields in the changelog schema.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor Grok 4.5

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🔍 Preview links for changed docs

• contribute-docs/content-types/changelogs.md

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✅ Elastic Docs Style Checker (Vale)

No issues found on modified lines!

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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.