Add elasticsearch_plugin fix to CHANGELOG

CHANGELOG.md

@@ -83,6 +83,7 @@ Ansible Changes By Release
 * reduced normal error redundancy and verbosity, display on increased and when needed
 * Give an informative error instead of a traceback if include_vars dir is file instead of directory (https://github.com/ansible/ansible/pull/31157)
 * Fix monit module's version check for color support (https://github.com/ansible/ansible/pull/31212)
+* Max `elasticsearch_plugin` module work with both 2.x and 5.x (https://github.com/ansible/ansible/issues/21989)
 
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