[NOTICE Update]Update year to 2021 (#4521)

NOTICE

@@ -1,5 +1,5 @@
 Apache DolphinScheduler (incubating)
-Copyright 2019-2020 The Apache Software Foundation
+Copyright 2019-2021 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).

dolphinscheduler-dist/release-docs/NOTICE

@@ -1,5 +1,5 @@
 Apache DolphinScheduler (incubating)
-Copyright 2019-2020 The Apache Software Foundation
+Copyright 2019-2021 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).