Updated copyright notices

Author: jstedfast

MailKit/Net/Imap/ImapTokenCache.cs

@@ -3,7 +3,7 @@
 //
 // Author: Jeffrey Stedfast <jestedfa@microsoft.com>
 //
-// Copyright (c) 2013-2023 .NET Foundation and Contributors
+// Copyright (c) 2013-2026 .NET Foundation and Contributors
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to deal