NuGetAudit severity bugs

[zivkan]

NuGet Product Used

dotnet.exe

Product Version

.NET 8 SDK previews

Worked before?

No response

Impact

I'm unable to use this version

Repro Steps & Context

There are some bugs regarding severity in the .NET 8 previews that introduced NuGetAudit

1. GitHub advisories are being reported at 1 level lower than they should be. For example, Newtonsoft.Json's advisory should be reported as high severity, but it's being reported as moderate.
2. The same off-by-1 error means that low severity advisories are never reported.
3. Similarly, using <NuGetAuditLevel> will filter out the github advisories of that level, since Audit thinks they're a lower severity than they really are.

Verbose Logs

No response