Bump dependency (fluent#896)

The previous version listed does not appear to be in the registry any longer. This is the latest release in the 7.x series of Elasticsearch.

Signed-off-by: Tom <[email protected]>

Author: petemounce

local-testing/logging-pipeline.md

@@ -56,7 +56,7 @@ services:
     depends_on:
       - elasticsearch
   elasticsearch:
-    image: elasticsearch:7.6.2
+    image: elasticsearch:7.17.6
     ports:
       - "9200:9200"
     environment: