Remove "What's new in C# 10"

[BillWagner]

Fixes #43290

As part of editing / affected files:

• Fixes Beware when mocking in unit tests! #30456
• Fixes Exception in pattern matching docs bears an incorrect message #36278
• Fixes Not only _record_ or DictionaryEntry beside any ValueTuple provides deconstructor #28090
• Fixes Extension methods for user-defined types vs Extension method for system types #28092
• Fixes Exception handling default for top-level statements #25607
• Fixes Struct initialization and default values – definitive assignment place #42331
• Fixes Struct initialization and default values – this access #42332
• Fixes Clarification may be needed #27405

Commit by commit:

• Remove references to "Beginning in C# 10", and the "What's new in C# 10" article.
• Fix warnings and add redirections
• Move Interpolated string handler tutorial out from the "new" section into the advanced section. Edit pass, and fix any outstanding issues.
• Move pattern matching tutorial from the "what's new" section to the general tutorial section.

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Internal previews

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📄 File	🔗 Preview link
docs/core/tutorials/top-level-templates.md	C# console app template generates top-level statements
docs/core/tutorials/with-visual-studio-code.md	Create a .NET console application using Visual Studio Code
docs/core/tutorials/with-visual-studio.md	Tutorial: Create a .NET console application using Visual Studio
docs/core/whats-new/dotnet-6.md	docs/core/whats-new/dotnet-6
docs/csharp/advanced-topics/performance/interpolated-string-handler.md	Tutorial: Write a custom string interpolation handler
docs/csharp/asynchronous-programming/async-return-types.md	Async return types
docs/csharp/fundamentals/functional/deconstruct.md	Deconstructing tuples and other types
docs/csharp/fundamentals/program-structure/top-level-statements.md	Top-level statements - programs without Main methods
docs/csharp/fundamentals/types/namespaces.md	docs/csharp/fundamentals/types/namespaces
docs/csharp/how-to/concatenate-multiple-strings.md	How to concatenate multiple strings (C# Guide)
docs/csharp/language-reference/attributes/general.md	"Attributes interpreted by the compiler: Miscellaneous"
docs/csharp/language-reference/builtin-types/default-values.md	Default values of C# types (C# reference)
docs/csharp/language-reference/builtin-types/nullable-reference-types.md	docs/csharp/language-reference/builtin-types/nullable-reference-types
docs/csharp/language-reference/builtin-types/record.md	Records (C# reference)
docs/csharp/language-reference/builtin-types/struct.md	Structure types (C# reference)
docs/csharp/language-reference/keywords/const.md	"The const keyword"
docs/csharp/language-reference/keywords/namespace.md	The namespace keyword
docs/csharp/language-reference/keywords/using-directive.md	The using directive
docs/csharp/language-reference/operators/addition-operator.md	Addition operators - + and +=
docs/csharp/language-reference/operators/lambda-expressions.md	docs/csharp/language-reference/operators/lambda-expressions
docs/csharp/language-reference/operators/patterns.md	Pattern matching - the is and switch expressions, and operators and, or, and not in patterns
docs/csharp/language-reference/tokens/interpolated.md	"$ - string interpolation - format string output"
docs/csharp/nullable-references.md	Nullable reference types
docs/csharp/programming-guide/classes-and-structs/local-functions.md	docs/csharp/programming-guide/classes-and-structs/local-functions
docs/csharp/toc.yml	Taken from https://github.com/dotnet/roslyn/wiki/Samples-and-Walkthroughs
docs/csharp/tour-of-csharp/index.yml	"C# documentation"
docs/csharp/tutorials/patterns-objects.md	Use pattern matching to build your class behavior for better code
docs/csharp/tutorials/records.md	docs/csharp/tutorials/records
docs/csharp/tutorials/top-level-statements.md	Tutorial: Explore ideas using top-level statements to build code as you learn
docs/csharp/whats-new/csharp-version-history.md	The history of C#

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