Added .NET 9 to test matrix

[Romfos]

🤔 What's changed?

• Added .NET 9 to test matrix
• LangVersion was updated to 13
• Update system.text.json to version 8 branch (because .NET 6 is EOL)

⚡️ What's your motivation?

.NET 9 will be GA in November
https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-9/overview

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)

♻️ Anything particular you want feedback on?

n\a

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[mpkorstanje]

LGTM. Cheers!