MM-53962: Adding ES8 dependency (#24399)

* Adding ES8 dependency

```release-note
NONE
```


Co-authored-by: Mattermost Build <build@mattermost.com>

server/tests/test-config.json

@@ -323,6 +323,7 @@
     },
     "ElasticsearchSettings": {
         "ConnectionUrl": "http://localhost:9200",
+        "Backend": "elasticsearch",
         "Username": "elastic",
         "Password": "changeme",
         "EnableIndexing": false,