Added .net 8 to test platforms

[Romfos]

🤔 What's changed?

• .NET sdk was updated to version 8
• Added .NET 8 to test platforms (new test platforms: .net 6, .net 7, .net 8)
• Use c# lang version 12

⚡️ What's your motivation?

.NET 8 soon will be released. This is LTS version and will be good to add it to testing platforms

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new behaviour)

📋 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]

Looks good overall. But can you rebase on/merge main?

[Romfos]

I would like to wait .net 8 release, this is a draft pr, but because you merged it - ok, lets start testing from preview version =)

[mpkorstanje]

My availability is a bit sparse these days. By the time I'll be able to look at this again there is a good chance 8.0 will have been released 😉.