C#: Add change note for NuGet CLI private registry filtering

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: michaelnebel

csharp/ql/lib/change-notes/2026-10-30-nuget-cli-private-registries.md

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+category: minorAnalysis
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+- The C# extractor continues to use the NuGet CLI to restore dependencies from `packages.config` files when private NuGet registries are configured, but excludes those private registries from the CLI's sources. Other applicable feeds remain available, while private registries continue to be used by `dotnet restore`.