Document what's new moniker exception

[jfversluis]

Summary

• Add guidance to avoid same-version moniker blocks in docs/whats-new/dotnet-*.md pages
• Update the instructions timestamp

Motivation

Versioned what's new pages should remain visible when readers arrive with an older view= value from the version selector.

Validation

• Ran git diff --check

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Internal previews

📄 File	🔗 Preview link
.github/copilot-instructions.md	.github/copilot-instructions

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

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[learn-build-service-prod]

Learn Build status updates of commit df7b90d:

✅ Validation status: passed

File	Status	Preview URL	Details
.github/copilot-instructions.md	✅Succeeded

For more details, please refer to the build report.