Build using .NET8.0 SDK

[mikkelbu]

This issue is extracted from two comments in #609

At the same time I wanted to move to .NET8.0 SDK to match NUnit.
However, the corresponding updated Roslyn Analyzer now insists on separating the CodeFix from the Analyzers.
CodeFixes should only be loaded by IDE, not by the build process.
#609 (comment)

and

In addition to that, unit tests fail due some other changes they made: GuOrg/Gu.Roslyn.Asserts#320
#609 (comment)