Update to v6.8.18

Author: Me-Phakorn

.env

@@ -1 +1 @@
-ELK_VERSION=7.13.1
+ELK_VERSION=6.8.18

docker-stack.yml

@@ -3,7 +3,7 @@ version: '3.3'
 services:
 
   elasticsearch:
-    image: docker.elastic.co/elasticsearch/elasticsearch:7.13.1
+    image: docker.elastic.co/elasticsearch/elasticsearch:6.8.18
     ports:
       - "9200:9200"
       - "9300:9300"
@@ -25,7 +25,7 @@ services:
       replicas: 1
 
   kibana:
-    image: docker.elastic.co/kibana/kibana:7.13.1
+    image: docker.elastic.co/kibana/kibana:6.8.18
     ports:
       - "5601:5601"
     configs: