[misc] mongodb: use the new db connector by default

mongojs was enabling it by default as well.

services/docstore/app/js/mongodb.js

@@ -1,7 +1,10 @@
 const Settings = require('settings-sharelatex')
 const { MongoClient, ObjectId } = require('mongodb')
 
-const clientPromise = MongoClient.connect(Settings.mongo.url)
+const clientPromise = MongoClient.connect(
+  Settings.mongo.url,
+  Settings.mongo.options
+)
 
 let setupDbPromise
 async function waitForDb() {

services/docstore/config/settings.defaults.js

@@ -14,7 +14,12 @@ const Settings = {
     }
   },
 
-  mongo: {},
+  mongo: {
+    options: {
+      useUnifiedTopology:
+        (process.env.MONGO_USE_UNIFIED_TOPOLOGY || 'true') === 'true'
+    }
+  },
 
   docstore: {
     backend: process.env.BACKEND || 's3',