docs: Add release notes for 3.6.9

[devin-ai-integration]

Adds release notes for the 3.6.9 patch release.

This release contains UI-v2 replatform work including new components for the flow run details page and automations wizard. All changes are categorized under "Development & Tidiness" as they are internal UI improvements.

Note: This release does not include a subtitle yet. A subtitle should be confirmed before merging (e.g., something related to the UI replatform theme).

Human Review Checklist

• Confirm whether a release subtitle should be added (previous releases have subtitles like "On Exception! On Timeout! On Error and Failure! 🦌")
• Verify all PRs merged since 3.6.8 are accounted for (PR Add prefect-kubernetes 0.7.2 release notes #19981 for prefect-kubernetes release notes was intentionally excluded per release notes guidelines)
• Confirm formatting matches existing release notes style

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

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Link to Devin run: https://app.devin.ai/sessions/cdf717e9eb804e349b39d7872470c4e5
Requested by: unknown ()

[devin-ai-integration]

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