[Automation] Update elastic stack version to 8.6.0-e4c15f15 for testi…

…ng (#1425)

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

testing/environments/snapshot.yml

@@ -3,7 +3,7 @@
 version: '2.3'
 services:
   elasticsearch:
-    image: docker.elastic.co/elasticsearch/elasticsearch:8.6.0-0fca2953-SNAPSHOT
+    image: docker.elastic.co/elasticsearch/elasticsearch:8.6.0-e4c15f15-SNAPSHOT
     # When extend is used it merges healthcheck.tests, see:
     # https://github.com/docker/compose/issues/8962
     # healthcheck:
@@ -42,7 +42,7 @@ services:
       - ./docker/logstash/pki:/etc/pki:ro
 
   kibana:
-    image: docker.elastic.co/kibana/kibana:8.6.0-0fca2953-SNAPSHOT
+    image: docker.elastic.co/kibana/kibana:8.6.0-e4c15f15-SNAPSHOT
     environment:
     - "ELASTICSEARCH_USERNAME=kibana_system_user"
     - "ELASTICSEARCH_PASSWORD=testing"