Update app-lib-dotnet to dotnet8 with c# 12

[ivarne]

We should ensure that v8 works with the newest dotnet version, which at the time of release of app-lib v8 will be net8.0.

If someone finds a compelling idea for why we would need multi-targeting net6.0 (or net7.0), I'm not against it, but forcing users to have similar setup and not have to ask (Oh, you run net6.0, we didn't properly test that) is a huge time saver in my opinion.

This PR does not include the fix of the update tool to change dotnet version, as it would conflict with changes in #329

Related Issue(s)

• No separate issue

Remarks

Something funny happened with FeatureManagment when updating, and I had to use the newest version to make the tests compile. Seems like a very sensible change.

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)
• All tests run green

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

No Coverage information
0.0% Duplication

[tjololo]

I can't see any reason for supporting multi targeting for v8 either