Added targets for net6.0 and net8.0 and allow RollForward

[FroggieFrog]

Description:

Fixes #447 Fixes #495

This PR adds the TargetFrameworks net6.0 and net8.0. Also as sugegsted in #419 the RollForward property was added.

I'm not sure about the piepline configurations, but adjusted them according to #419

Checklist:

• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] I have tested my changes by running the extension in VS2017
• [x] I have tested my changes by running the extension in VS2019
• [x] I have tested my changes by running the extension in VS2022
• [x] If changes to the documentation are needed, I have noted this in the description above

[renebentes]

Any update on this? I would really like to use it without installing .NET 6

[Jeanjean]

Me too

[Arlodotexe]

I'd like to see an upgrade to net8.0 as well. CommunityToolkit/Windows relies on this.