[Automation] Update elastic stack version to 8.6.0-8a615646 for testing (#2050)

Co-authored-by: apmmachine <infra-root-apmmachine@elastic.co>

Author: apmmachine

dev-tools/integration/.env

@@ -1,4 +1,4 @@
-ELASTICSEARCH_VERSION=8.6.0-4765d2b0-SNAPSHOT
+ELASTICSEARCH_VERSION=8.6.0-8a615646-SNAPSHOT
 ELASTICSEARCH_USERNAME=elastic
 ELASTICSEARCH_PASSWORD=changeme
 TEST_ELASTICSEARCH_HOSTS=localhost:9200