Fix user renaming logic

Jellyfin.Server.Implementations/Users/UserManager.cs

@@ -147,7 +147,7 @@ namespace Jellyfin.Server.Implementations.Users
                 throw new ArgumentException("The new and old names must be different.");
             }
 
-            if (Users.Any(u => u.Id != user.Id && u.Username.Equals(newName, StringComparison.Ordinal)))
+            if (Users.Any(u => u.Id != user.Id && u.Username.Equals(newName, StringComparison.OrdinalIgnoreCase)))
             {
                 throw new ArgumentException(string.Format(
                     CultureInfo.InvariantCulture,