Update app-lib-dotnet to multi-target dotnet6 and dotnet8 with c# 12

[ivarne]

Also include code that allows for unchecked use of required attributes in net6.0 (so we can start using required properties without dropping support for net6.0)

This is an alternative to #340 that preserves support for apps that use net6.0. I think this adds much more complexity to app management than what it is worth.

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)

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

0.0% Coverage
0.0% Duplication