Update .NET dependencies to C# 13.0 and .NET 9.0

[Copilot]

This PR updates the project dependencies to use the latest stable versions of .NET and C# while maintaining compatibility and ensuring no tests break.

Changes

Updated three key configuration files to modernize the .NET stack:

• global.json: SDK version updated from 8.0.0 → 9.0.0
• Directory.Build.props: LangVersion updated from 12.0 → 13.0 (C# 13)
• Common.props: TargetFrameworks updated from net8.0;net7.0;net6.0 → net9.0;net8.0

Updated documentation to reflect the new versions:

• README.md: Updated requirements section from ".NET 8.0, 7.0, and 6.0" to ".NET 9.0 and 8.0"
• Errata.md: Updated title from "Essential C# 12.0" to "Essential C# 13.0"
• .github/actions/setup-dotnet/action.yml: Updated dotnet-version list from 6.0.x, 7.0.x, 8.0.x to 8.0.x, 9.0.x

Rationale

• Removes EOL framework warnings: .NET 6.0 and 7.0 are out of support and were generating NETSDK1138 warnings
• Enables C# 13.0 features: Projects can now use the latest C# language features
• Reduces multi-targeting overhead: Simplified from 3 to 2 target frameworks while maintaining modern platform support
• Maintains compatibility: Kept .NET 8.0 as a secondary target for broader compatibility

Validation

✅ Build: Solution builds successfully with 0 errors
✅ Tests: All tests pass across both target frameworks (net9.0 and net8.0)
✅ Warnings: No new warnings introduced; only expected warnings remain (intentional #warning directives and SYSLIB warnings for code examples)
✅ SDK: Verified using .NET 9.0.305 SDK

All existing tests continue to pass, confirming no breaking changes were introduced.

Original prompt

Update the .net dependencies as much as we can without

1. Breaking out tests
2. Causing any build warnings or errors

You can update TFM's and LangVersion (and global.json) - but only to C# 13.0 and it's compatible .net version (I think .net 9.0?)

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[Copilot]

Pull Request Overview

Updates .NET dependencies to use the latest stable versions (C# 13.0 and .NET 9.0) while removing end-of-life framework targets to eliminate warnings and modernize the codebase.

• Updated SDK version from 8.0.0 to 9.0.0
• Upgraded C# language version from 12.0 to 13.0
• Simplified target frameworks from net8.0;net7.0;net6.0 to net9.0;net8.0

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
global.json	Updates .NET SDK version to 9.0.0 for latest tooling
Directory.Build.props	Enables C# 13.0 language features across all projects
Common.props	Removes EOL frameworks and targets .NET 9.0 and 8.0

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[BenjaminMichaelis]

@copilot did we update any relevant documentation and similar?

[Copilot]

@copilot did we update any relevant documentation and similar?

Yes, I've updated the documentation in commit f5f79e2:

• README.md: Updated requirements section to reflect .NET 9.0 and 8.0
• Errata.md: Updated title from "Essential C# 12.0" to "Essential C# 13.0"
• .github/actions/setup-dotnet/action.yml: Updated dotnet-version list from 6.0.x/7.0.x/8.0.x to 8.0.x/9.0.x