[.NET] Update target frameworks

[Romfos]

🤔 What's changed?

• Drop .NET 7 from test matrix
• Replace .NET 6 to .NET 8. because .NET 6 will reach End of Support on November 12, 2024
• Update C# language version to 12

⚡️ What's your motivation?

• .NET 7 is out of support. https://devblogs.microsoft.com/dotnet/dotnet-7-end-of-support/
• .NET 6 (LTS) will reach End of Support on November 12, 2024 https://devblogs.microsoft.com/dotnet/dotnet-6-end-of-support/

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)
• 💥 Breaking change (incompatible changes to the API)

♻️ Anything particular you want feedback on?

📋 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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[Romfos]

please, squash PR during the merge

[mpkorstanje]

Cheers. I'm waiting for a review from either @gasparnagy or @clrudolphi.

Context: cucumber/messages#237 (comment)

[gasparnagy]

Thx. I'm OK with this changes.

[gasparnagy]

@mpkorstanje these changes are "technically" breaking changes, but in practice they will not break any code, because we still have the support for ".NET Standard 2.0" which anyway covers all those older frameworks that are now removed from the "direct" support.