dotnet: CONTRIBUTING is outdated

[304NotModified]

I think https://github.com/cucumber/gherkin/blob/main/dotnet/CONTRIBUTING.md is outdated.

• .NET 5 is End of Life (EOL) long ago - 10 May 2022
• The links links to .NET Core 2 (even more EOL)
• The projects are using .NET 8, .NET 9 and .NET 4.8.1

I don't know MacOS/Linux could work with .NET 4.8.1, so I think the advices is to install SDK 9?

[mpkorstanje]

Please feel free to update it!

I'm not familiar with the common practices around .Net, but I imagine that they don't change too much.

If .Net version numbers are required, you can reference the GitHub action. That's less likely to go out of date.

https://github.com/cucumber/gherkin/blob/main/.github/workflows/test-dotnet.yml