Remove 6.0.2 ref packs, add back 8.0.0

[wtgodbe]

Should unblock dotnet/sdk#47509

[MichaelSimons]

FYI, I removed the KnownFrameworkReference handling for 6.0.2

[ViktorHofer]

I would add a KnownFrameworkReference into src/referencePackages/Directory.Build.targets for aspnetcore (in case a package gets added here that depends on it):

<KnownFrameworkReference Update="@(KnownFrameworkReference->WithMetadataValue('Identity', 'Microsoft.AspNetCore.App'))">
      <TargetingPackVersion Condition="$([System.Text.RegularExpressions.Regex]::IsMatch('%(TargetFramework)', '^net\d+\.0$'))">$([System.Text.RegularExpressions.Regex]::Match('%(TargetFramework)', '\d+').Value).0.0</TargetingPackVersion>
    </KnownFrameworkReference>