Add documentation for backoffice authentication updates

[kjac]

📋 Description

This adds the breaking changes from umbraco/Umbraco-CMS#20820 (which is a continuation of umbraco/Umbraco-CMS#20779) to the release specific upgrade notes for V17.

📎 Related Issues (if applicable)

None

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

V17.

Deadline (if relevant)

As soon as possible.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[github-actions]

The Link Checker found broken links in your PR.
Please review the following list:

Broken links found in: 17/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

⚠ Unknown error → [503] https://github.com/umbraco/UmbracoDocs/blob/umbraco-eol-versions/11/umbraco-forms/installation/version-specific.md |#124; Rejected status code (this depends on your "accept" configuration): Service Unavailable

[eshanrnh]

Looks great, @kjac 🙌