Update dependencies and target frameworks and fix build warnings

[mus65]

Starting with .NET 9, NuGet Audit will transitively scan for CVEs, so consuming applications will get a warning about System.Text.Json:

warning NU1903: Package 'System.Text.Json' 6.0.1 has a known high severity vulnerability, GHSA-8g4q-xg66-9fp4

#24 already optimized this to only affect .NET Framework (but it's still not released).

This updates all dependencies (except Nunit 4 which has a lot of breaking changes) and cleans up some build warnings:

• removed netstandard2.0 from test projects. This doesn't make sense because you can't run tests against netstandard2.0 (LangVersion also isn't needed anymore then)
• removed EOL net7.0 target and added net8.0
• removed Microsoft.CodeAnalysis.NetAnalyzersbecause it's part of the SDK since .NET 5

[xfischer]

Hi ! Thanks a lot for your contribution. Will review and ask a review from @janusw (I have to add it here also) who did so much lately for the main repo.
Please allow a couple of hours as I don't have access to my laptop now.

[janusw]

I don't see anything wrong with this, except that probably the GHA hooks need to be updated to deal with .NET 8 as well. Maybe someone can approve the workflows to run, so that we can see them fail? 😆

[mus65]

Good point, I also updated the GitHub actions and dropped the EOL .NET versions from CI.

[janusw]

Good point, I also updated the GitHub actions and dropped the EOL .NET versions from CI.

👍 Now we only need someone to approve the workflows so that they will actually run. @xfischer Can you do that?

[janusw]

Alright, so the build job runs fine, but the benchmarking job fails. That one also needs to be adjusted:
https://github.com/GeoJSON-Net/GeoJSON.Text/blob/main/.github/workflows/continous-benchmark.yml

[xfischer]

@janusw @mus65 thanks to both of you.
@janusw I've added you as a maintainer

[mus65]

Benchmarks should hopefully be fixed now, but I guess the workflow needs another approval. 😄 I removed net6.0 from the benchmark project since .NET 6 is EOL next month anyway.

[janusw]

Benchmarks should hopefully be fixed now, but I guess the workflow needs another approval. 😄

Since I'm a maintainer, I can do that now. 💪 (thanks, @xfischer !)

[janusw]

Alright, all the CI jobs run fine by now and the PR was approved already, so I'll just merge it ...