Fix NuGetConfigUpdater removing all credentials for preview builds

eng/update-dependencies/NuGetConfigUpdater.cs

@@ -89,6 +89,12 @@ private static string ToStringWithDeclaration(XDocument doc)
         return builder.ToString();
     }
 
+    /// <summary>
+    /// Updates the packageSourceCredentials section of the NuGet.config for the current version's package source.
+    /// Credentials are only needed for internal, non-public-preview builds which use authenticated feeds.
+    /// Public preview builds use public feeds, so their credentials are removed if present.
+    /// Only the current version's entry is modified — other versions' credentials are left intact.
+    /// </summary>
     private void UpdatePackageSourceCredentials(DotNetVersion sdkVersion, string pkgSrcName, XElement configuration)
     {
         XElement? pkgSourceCreds = configuration.Element("packageSourceCredentials");
@@ -108,7 +114,14 @@ private void UpdatePackageSourceCredentials(DotNetVersion sdkVersion, string pkg
         }
         else
         {
-            pkgSourceCreds?.Remove();
+            // Only remove the credentials entry for the current version's package source,
+            // leaving credentials for other versions intact (e.g. a non-preview version
+            // may still need authenticated access even if this preview version doesn't).
+            pkgSourceCreds?.Element(pkgSrcName)?.Remove();
+            if (pkgSourceCreds is not null && !pkgSourceCreds.HasElements)
+            {
+                pkgSourceCreds.Remove();
+            }
         }
     }