[Chore] update notice for 2026 (#640)

Author: JNSimba

NOTICE.txt

@@ -1,5 +1,5 @@
 Flink Connector for Apache Doris
-Copyright 2018-2025 The Apache Software Foundation
+Copyright 2018-2026 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).