Align with .NET 8.0 SDK analyzers

[idg10]

We're temporarily disabling a couple of diagnostics to ensure the project continues to build. (For some reason, the use of <AnalysisLevel>7.0</AnalysisLevel> isn't quite enough to prevent new messages when building on the .NET 8.0 SDK.)

However, our intention is to build cleanly without such workarounds on the latest SDK, so this needs to be addressed properly. So in addition to updating that AnalysisLevel build variable from 7.0 to 8.0, we also need to remove the <NoWarn> element (and associated comment) that follows it.