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

mongojs was enabling it by default as well.

services/document-updater/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
+)
 
 async function healthCheck() {
   const internalDb = (await clientPromise).db()

services/document-updater/config/settings.defaults.js

@@ -174,6 +174,10 @@ module.exports = {
   dispatcherCount: process.env.DISPATCHER_COUNT,
 
   mongo: {
+    options: {
+      useUnifiedTopology:
+        (process.env.MONGO_USE_UNIFIED_TOPOLOGY || 'true') === 'true'
+    },
     url:
       process.env.MONGO_CONNECTION_STRING ||
       `mongodb://${process.env.MONGO_HOST || '127.0.0.1'}/sharelatex`