Packaging gives warning NU5048

[ghost]

We get this warning for each NuGet package we create:

C:\Program Files\dotnet\sdk\3.0.100-preview9-014004\Sdks\NuGet.Build.Tasks.Pack\buildCrossTargeting\NuGet.Build.Tasks.Pack.targets(198,5):
error NU5048: The 'PackageIconUrl'/'iconUrl' element is deprecated.
Consider using the 'PackageIcon'/'icon' element instead.
Learn more at https://aka.ms/deprecateIconUrl
[C:\dev\sarif\sarif-sdk\src\Sarif.Driver\Sarif.Driver.csproj]

The error message tells the story and the instructions at https://aka.ms/deprecateIconUrl tell how to fix it. We can't refer to the icon through a URL; we need to store it locally and refer to it with the <PackageIcon> element. It turns out we already have a copy of the icon triskelion.png in the repo; we just weren't using it.

See also NuGet/Home#8569

[ghost]

@michaelcfanning FYI. I fixed this in the same change as the work I'm doing for #1673.