Update snippet TFMs from .NET 6 to .NET 8

[gewarren]

Replaces #43721.

Contributes to #43161.

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Internal previews

📄 File	🔗 Preview link
docs/core/tutorials/library-with-visual-studio-code.md	Tutorial: Create a .NET class library using Visual Studio Code
docs/core/tutorials/library-with-visual-studio.md	Create a .NET class library using Visual Studio
docs/core/tutorials/testing-library-with-visual-studio-code.md	Tutorial: Test a .NET class library using Visual Studio Code
docs/core/tutorials/with-visual-studio.md	docs/core/tutorials/with-visual-studio
docs/fundamentals/runtime-libraries/system-byte.md	System.Byte struct
docs/standard/base-types/formatting-types.md	Overview: How to format numbers, dates, enums, and other types in .NET
docs/standard/serialization/binaryformatter-event-source.md	BinaryFormatter event source
docs/standard/threading/cancellation-in-managed-threads.md	Cancellation in Managed Threads