Update NuGet-6.12.md

[icnocop]

Promoting a warning to an error is a breaking change.

NuGet/Home#13289 is also labeled with the category BreakingChange.

[learn-build-service-prod]

Learn Build status updates of commit df668cb:

💡 Validation status: suggestions

File	Status	Preview URL	Details
docs/release-notes/NuGet-6.12.md	💡Suggestion	View	Details

docs/release-notes/NuGet-6.12.md

• Line 2, Column 1: [Suggestion: ms-date-missing - See documentation] Missing required attribute: 'ms.date'.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

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[nkolev92]

Hey @icnocop, I fixed it in 06a4c91.