Updating NuGet package to use <icon> instead of deprecated <iconUrl> +

+ Needed to update nuget.exe to the (latest) 5.3 for that
+ What results in need for dependency groups
+ What results in need for explicit framework versions in target folder, otherwise nuget issues confusing warnings

Source commit: 5ba04abf35d20a1c98c6b55a7f2347a94b14ea5e

deployment/WinSCPnet.nuspec

@@ -8,7 +8,7 @@
     <owners>Martin Prikryl</owners>
     <license type="expression">MPL-2.0</license>
     <projectUrl>https://winscp.net/eng/docs/library</projectUrl>
-    <iconUrl>https://winscp.net/pad/winscp64.png</iconUrl>
+    <icon>winscp64.png</icon>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <summary>The WinSCP .NET assembly is a .NET wrapper around WinSCP’s scripting interface.</summary>
     <description>The WinSCP .NET assembly is a .NET wrapper around WinSCP’s scripting interface that allows your code to connect to a remote machine and manipulate remote files over SFTP, FTP, WebDAV, S3 and SCP sessions.
@@ -20,11 +20,16 @@ For documentation and examples of use, see project website.
 The NuGet package includes the assembly itself and a required WinSCP executable. When installed, it adds the assembly as reference to your project and sets up WinSCP executable to be copied to project output directory, so that it can be found on run-time.</description>
     <copyright>Copyright © 2012-2019 Martin Prikryl</copyright>
     <tags>winscp sftp ftp ftps webdav s3 scp transfer</tags>
+    <dependencies>
+      <group targetFramework=".NETStandard2.0"/>
+      <group targetFramework=".NETFramework4.0"/>
+    </dependencies>
   </metadata>
   <files>
-    <file src="$DotNetBuildConfigDir$\net40\WinSCPnet.dll" target="lib\net"/>
-    <file src="$DotNetBuildConfigDir$\netstandard2.0\WinSCPnet.dll" target="lib\netstandard"/>
+    <file src="$DotNetBuildConfigDir$\net40\WinSCPnet.dll" target="lib\net40"/>
+    <file src="$DotNetBuildConfigDir$\netstandard2.0\WinSCPnet.dll" target="lib\netstandard2.0"/>
     <file src="$BuildConfigDir$\WinSCP.exe" target="tools"/>
     <file src="WinSCP.targets" target="build"/>
+    <file src="winscp64.png" target=""/>
   </files>
 </package>