fix: Upgrade docker-compose.yml to ES 7.17.25

Not all tests ran green with ElasticSearch 7.4.2. As it's pretty old, I
think it's fair to move on and develop with latest stable (which most
people will use anyways.)

docker-compose.yml

@@ -1,7 +1,7 @@
 version: "2"
 services:
   elasticsearch1:
-    image: docker.elastic.co/elasticsearch/elasticsearch:7.4.2
+    image: docker.elastic.co/elasticsearch/elasticsearch:7.17.25
     container_name: elasticsearch1
     environment:
       - node.name=elasticsearch1
@@ -21,7 +21,7 @@ services:
       - esnet
 
   elasticsearch2:
-    image: docker.elastic.co/elasticsearch/elasticsearch:7.4.2
+    image: docker.elastic.co/elasticsearch/elasticsearch:7.17.25
     environment:
       - node.name=elasticsearch2
       - cluster.name=docker-cluster