Update copyright in SqlColumnCollection.cs

Author: SergeiPavlov

Orm/Xtensive.Orm/Sql/Dml/Collections/SqlColumnCollection.cs

@@ -1,6 +1,6 @@
-// Copyright (C) 2003-2010 Xtensive LLC.
-// All rights reserved.
-// For conditions of distribution and use, see license.
+// Copyright (C) 2008-2021 Xtensive LLC.
+// This code is distributed under MIT license terms.
+// See the License.txt file in the project root for more information
 
 using System;
 using System.Collections;