ZOOKEEPER-4056: Update copyright notices from 2020 to 2021

See ZOOKEEPER-3745 for previous iteration.

Author: Damien Diederen <dd@crosstwine.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>

Closes #1577 from ztzg/ZOOKEEPER-4056-copyright-2021

NOTICE.txt

@@ -1,5 +1,5 @@
 Apache ZooKeeper
-Copyright 2009-2020 The Apache Software Foundation
+Copyright 2009-2021 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).

zookeeper-docs/src/main/resources/markdown/zookeeperCLI.md

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2020 The Apache Software Foundation
+Copyright 2002-2021 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

zookeeper-docs/src/main/resources/markdown/zookeeperMonitor.md

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2020 The Apache Software Foundation
+Copyright 2002-2021 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

zookeeper-docs/src/main/resources/markdown/zookeeperTools.md

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2020 The Apache Software Foundation
+Copyright 2002-2021 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

zookeeper-docs/src/main/resources/markdown/zookeeperUseCases.md

@@ -1,5 +1,5 @@
 <!--
-Copyright 2002-2020 The Apache Software Foundation
+Copyright 2002-2021 The Apache Software Foundation
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.

zookeeper-server/src/main/resources/NOTICE.txt

@@ -1,5 +1,5 @@
 Apache ZooKeeper
-Copyright 2009-2020 The Apache Software Foundation
+Copyright 2009-2021 The Apache Software Foundation
 
 This product includes software developed at
 The Apache Software Foundation (http://www.apache.org/).