Publish What's new for .NET 9 Preview 6

[gewarren]

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📄 File	🔗 Preview link
docs/core/compatibility/sdk/8.0/dotnet-restore-audit.md	'dotnet restore' produces security vulnerability warnings
docs/core/diagnostics/metrics-instrumentation.md	Creating Metrics
docs/core/tools/buildcheck-rules/bc0101.md	BC0101
docs/core/tools/buildcheck-rules/bc0102.md	docs/core/tools/buildcheck-rules/bc0102
docs/core/tools/buildcheck-rules/index.md	BuildCheck rule list
docs/core/tools/dotnet-restore.md	dotnet restore command
docs/core/whats-new/dotnet-8/sdk.md	What's new in the SDK and tooling for .NET 8
docs/core/whats-new/dotnet-9/libraries.md	docs/core/whats-new/dotnet-9/libraries
docs/core/whats-new/dotnet-9/overview.md	What's new in .NET 9
docs/core/whats-new/dotnet-9/runtime.md	What's new in .NET 9 runtime
docs/core/whats-new/dotnet-9/sdk.md	What's new in the SDK for .NET 9
docs/standard/base-types/best-practices-regex.md	Best practices for regular expressions in .NET
docs/standard/memory-and-spans/index.md	"Memory-related and span types"
docs/visual-basic/programming-guide/concepts/linq/how-to-combine-linq-queries-with-regular-expressions.md	How to combine LINQ queries with regular expressions (Visual Basic)
docs/visual-basic/programming-guide/concepts/linq/how-to-query-the-contents-of-files-in-a-folder-linq.md	docs/visual-basic/programming-guide/concepts/linq/how-to-query-the-contents-of-files-in-a-folder-linq