Update ELK Stack Docker tags to v6.8.21 (main) (patch)

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
docker.elastic.co/elasticsearch/elasticsearch	final	patch	6.8.20 -> 6.8.21
docker.elastic.co/kibana/kibana	final	patch	6.8.20 -> 6.8.21
docker.elastic.co/logstash/logstash	final	patch	6.8.20 -> 6.8.21

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Release Notes

elastic/elasticsearch

v6.8.21

Compare Source

Downloads: https://elastic.co/downloads/elasticsearch
Release notes: https://www.elastic.co/guide/en/elasticsearch/reference/6.8/release-notes-6.8.21.html

elastic/kibana

v6.8.21

Compare Source

Downloads: https://elastic.co/downloads/kibana
Release notes: https://www.elastic.co/guide/en/kibana/6.8/release-notes-6.8.21.html

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Configuration

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