Revise C# Style Guide with new formatting rules

[glennawatson]

Update C# Style Guide to Incorporate Modern C# Features (C# 8-12)

This commit updates the C# Style Guide to align with modern C# features from versions 8 through 12, enhancing code readability, maintainability, and safety. Key changes include:

• New Content: Added guidelines for modern C# features, including nullable reference types, ranges/indices, using declarations, switch expressions, records, init-only setters, target-typed new expressions, file-scoped namespaces, raw string literals, required members, primary constructors, collection expressions, and more (items 21–36).
• Better Readability: Structured new guidelines to be concise and clear, maintaining consistency with existing rules for Allman style, indentation, and naming conventions.
• Content Correction: Updated references to reflect current Microsoft documentation and best practices, ensuring accuracy for newer C# features.
• Improved Messaging: Emphasized the use of modern features to reduce boilerplate, enhance immutability, and improve type safety, while maintaining compatibility with existing style preferences.

These changes ensure the style guide remains relevant for current C# development while preserving the original structure and intent. The analyzer ruleset is recommended for updates to enforce new features like nullable reference types.

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