Nuget packages should be conform to newest spec

[pde-cds]

While uploading Release 1.14.3 of SharpSVN to nuget.org, there were serverall problems which should be fixed for next version.

• [x] iconUrl is deprecated, use instead (https://learn.microsoft.com/de-de/nuget/reference/nuspec#icon) -> Removed iconUrl because License of the Icon is not clear
• [x] readme is mandatory (https://learn.microsoft.com/de-de/nuget/reference/nuspec#icon)
• [x] symbols.nupgk needs to be in the new .snupgk format (-SymbolPackageFormat snupkg in nuspec\build.cmd)