Set 6.8.21 as default version

The Elasticsearch 6.8.21 package uses a log4j version that CVE 2021-44228
security issue.

Author: Miguel Angel

defaults/main.yml

@@ -4,7 +4,7 @@
 elasticsearch_user: elasticsearch
 elasticsearch_group: elasticsearch
 elasticsearch_download_url: https://artifacts.elastic.co/downloads/elasticsearch
-elasticsearch_version: "6.8.3"
+elasticsearch_version: "6.8.21"
 elasticsearch_apt_repos: []
 elasticsearch_apt_java_package: openjdk-8-jre-headless
 elasticsearch_java_home: "/usr/lib/jvm/java-1.8.0-openjdk-amd64"