Upgrade ElasticSearch to 6.8.9 (glitch-soc#1348)

Since tootsuite#13828, ElasticSearch has been failing to work
properly, throwing the following:

```
Elasticsearch::Transport::Transport::Errors::BadRequest ([400] {"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"request [/tags/tag/_search] contains unrecognized parameter: [rest_total_hits_as_int]"}],"type":"illegal_argument_exception","reason":"request [/tags/tag/_search] contains unrecognized parameter: [rest_total_hits_as_int]"},"status":400})
```

This commit is based entirely on:
lo48576@1209f3e

docker-compose.yml

@@ -25,7 +25,7 @@ services:
 
   es:
     restart: always
-    image: docker.elastic.co/elasticsearch/elasticsearch-oss:6.1.3
+    image: docker.elastic.co/elasticsearch/elasticsearch-oss:6.8.9
     environment:
       - bootstrap.memory_lock=true
       - "ES_JAVA_OPTS=-Xms512m -Xmx512m"