automation: Update docker-mastodon_elasticsearch Docker tag to v8.15.4

This MR contains the following updates:

Package	Update	Change
docker-mastodon_elasticsearch (source)	patch	8.15.3 -> 8.15.4

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Release Notes

elastic/elasticsearch (docker-mastodon_elasticsearch)

v8.15.4: Elasticsearch 8.15.4

Compare Source

Downloads: https://elastic.co/downloads/elasticsearch Release notes: https://www.elastic.co/guide/en/elasticsearch/reference/8.15/release-notes-8.15.4.html

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