Remove net6 / Label Pubsub Package as Release / Update Nuget Packages

[romanett]

Proposed changes

This PR removes the target for the outdated .net6 Runtime.
Also the CI Pipelines are updated to use Net8 instead of net6 for tests.
Starting with this PR the Pubsub package is not labelled beta any more.
Update Nuget Packages in the Fuzzing Project.
Also an explicit target for Visual Studio 2019 is removed.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.