Fix ProjectMigrateErrorMessage to correctly describe packages.config to PackageReference migration

Co-authored-by: donnie-msft <[email protected]>

Author: Copilot

src/NuGet.Clients/NuGet.Tools/Resources.resx

@@ -151,7 +151,7 @@
     <value>The project '{0}' is unsupported</value>
   </data>
   <data name="ProjectMigrateErrorMessage" xml:space="preserve">
-    <value>Project is not eligible for migration as it's not project.json based.</value>
+    <value>Project is not eligible for migration. Either the project is not packages.config based or doesn't support PackageReference yet. Visit https://docs.microsoft.com/en-us/nuget/reference/migrate-packages-config-to-package-reference for more information.</value>
   </data>
   <data name="ProjectJsonMigrateErrorMessage" xml:space="preserve">
     <value>Project is not eligible for migration. Either the project is not packages.config based or doesn't support PackageReference yet. Visit https://docs.microsoft.com/en-us/nuget/reference/migrate-packages-config-to-package-reference for more information.</value>