Use ToLowerInvariant to avoid locale issues. (#7967)

Author: ryuyu

src/NuGetGallery.Services/Authentication/ApiKeyV4.cs

@@ -143,7 +143,7 @@ private bool TryParseInternal(string plaintextApiKey)
 
         private string Normalize(string input)
         {
-            return input.ToLower();
+            return input.ToLowerInvariant();
         }
     }
 }