Update deprecated nuspec elements

[rprouse]

λ => nuget pack nunit-template.nuspec
Attempting to build package from 'nunit-template.nuspec'.
Successfully created package 'C:\Src\NUnit\dotnet-new-nunit\NUnit3.DotNetNew.Template.1.9.0.nupkg'.
WARNING: NU5125: The 'licenseUrl' element will be deprecated. Consider using the 'license' element instead.
WARNING: NU5048: The 'PackageIconUrl'/'iconUrl' element is deprecated. Consider using the 'PackageIcon'/'icon' element instead. Learn more at https://aka.ms/deprecateIconUrl