Update docker.elastic.co/elasticsearch/elasticsearch Docker tag to v6.8.23

Author: renovate[bot]

docker-compose.yml

@@ -16,7 +16,7 @@ services:
         source: .
         target: /timdex
   elasticsearch:
-    image: docker.elastic.co/elasticsearch/elasticsearch:6.8.4
+    image: docker.elastic.co/elasticsearch/elasticsearch:6.8.23@sha256:ca5dac2352e09263e55cccac445d69a0d2eec9c8c26a4c95c3b1430de1f427f2
     environment:
       - discovery.type=single-node
       - xpack.security.enabled=false