automation: Update mastodon-elasticsearch Docker tag to v7.17.21

This MR contains the following updates:

Package	Update	Change
mastodon-elasticsearch (source)	patch	7.17.19 -> 7.17.21

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Release Notes

elastic/elasticsearch (mastodon-elasticsearch)

v7.17.21: Elasticsearch 7.17.21

Compare Source

Downloads: https://elastic.co/downloads/elasticsearch Release notes: https://www.elastic.co/guide/en/elasticsearch/reference/7.17/release-notes-7.17.21.html

v7.17.20: Elasticsearch 7.17.20

Compare Source

Downloads: https://elastic.co/downloads/elasticsearch Release notes: https://www.elastic.co/guide/en/elasticsearch/reference/7.17/release-notes-7.17.20.html

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