Update .NET tool nuspec properties (#567)

Update nuspec Properties to avoid warnings:

NU5125: The 'licenseUrl' element will be deprecated. Consider using the
'license' element instead.
NU5048: The 'PackageIconUrl'/'iconUrl' element is deprecated. Consider
using the 'PackageIcon'/'icon' element instead. Learn
The package <> is missing a readme. Go to
https://aka.ms/nuget/authoring-best-practices/readme to learn why
package readmes are important.

Author: embetten

CredentialProvider.Microsoft/Microsoft.Artifacts.CredentialProvider.NuGet.Tool.nuspec

@@ -7,20 +7,22 @@
     <authors>Microsoft</authors>
      <owners>microsoft,nugetvss</owners>
     <description>The Microsoft Credential Provider tool enables the .NET CLI to authenticate with Azure Artifacts feeds. Use this tool to seamlessly restore and publish packages from secured feeds. For more information, see: http://docs.nuget.org/Consume/Credential-Providers</description>
-    <licenseUrl>http://go.microsoft.com/fwlink/?LinkId=329770</licenseUrl>
+    <license type="expression">MIT</license>
     <projectUrl>http://go.microsoft.com/fwlink/?LinkId=623217</projectUrl>
-    <iconUrl>http://go.microsoft.com/fwlink/?LinkId=615568</iconUrl>
+    <icon>nupkg-icon.png</icon>
     <requireLicenseAcceptance>true</requireLicenseAcceptance>
     <copyright>© Microsoft Corporation. All rights reserved.</copyright>
     <tags>Microsoft Visual Studio Team Services VSTS VSO VisualStudio Package Management NuGet</tags>
     <packageTypes>
       <packageType name="DotnetTool" />
     </packageTypes>
-
+    <readme>..\README.md</readme>
   </metadata>
     <files>
+       <file src="nupkg-icon.png" target="\" />
+       <file src="..\README.md" target="\" />
        <file src="ThirdPartyNotices.txt" target="\" />
        <file src="EULA_Microsoft Visual Studio Team Services Credential Provider.docx" target="\" />
-       <file src="bin\$Configuration$\net8.0\publish\**\*.*" target="tools\net8.0\any" />
+       <file src="bin\$Configuration$\net8.0\**\*.*" target="tools\net8.0\any" />
     </files>
 </package>