tests: elasticsearch uses wrong version in docker-compose

[powersj]

Relevant telegraf.conf

n/a

Logs from Telegraf

--- FAIL: TestElasticsearchQuery (0.01s)
    elasticsearch_query_test.go:583: 
        	Error Trace:	elasticsearch_query_test.go:583
        	            				once.go:68
        	            				once.go:59
        	            				elasticsearch_query_test.go:581
        	Error:      	Received unexpected error:
        	            	elasticsearch version 7.2.0 not supported (currently supported versions are 5.x and 6.x)
        	Test:       	TestElasticsearchQuery
--- FAIL: TestElasticsearchQuery_getMetricFields (0.02s)
    elasticsearch_query_test.go:656: 
        	Error Trace:	elasticsearch_query_test.go:656
        	Error:      	Received unexpected error:
        	            	elasticsearch version 7.2.0 not supported (currently supported versions are 5.x and 6.x)
        	Test:       	TestElasticsearchQuery_getMetricFields
FAIL
FAIL	github.com/influxdata/telegraf/plugins/inputs/elasticsearch_query	0.116s

System info

master

Docker

No response

Steps to reproduce

1. docker-compose up
2. go test -count=1 -race -timeout 30s ./plugins/inputs/elasticsearch_query/...

Expected behavior

tests use a supported version of elasticsearch

Actual behavior

tests fail the version check

Additional info

-    image: docker.elastic.co/elasticsearch/elasticsearch:7.2.0
+    image: docker.elastic.co/elasticsearch/elasticsearch:6.8.23