Update packaging to use dotnet pack with centralized MSBuild properties

[Copilot]

• Migrate from nuget.exe to dotnet pack with MSBuild package properties
• Create Directory.Build.props with centralized common package properties
• Update BuildNugets.ps1 to use dotnet pack instead of nuget.exe
• Remove unnecessary build step from packaging script
• Refactor common package properties to Directory.Build.props
• Remove empty Package Content sections from project files

Benefits Achieved:

• Eliminates external dependency on nuget.exe tool
• Single source of truth for package metadata in MSBuild properties
• Better integration with standard .NET CLI and MSBuild tooling
• Simplified workflow with integrated pack process
• Automatic dependency resolution from PackageReference and ProjectReference
• Reduced duplication through centralized Directory.Build.props
• Clean project files without unnecessary empty sections

Migration Complete:
All packaging has been successfully migrated from .nuspec files to MSBuild properties while maintaining backwards compatibility for package consumers.

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[Keboo]

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