enable gitlab automerge by default

This patch changes the automerge config to use the gitlab native feature
of automerging changing using the "merge if CI pipeline success"
feature. This should prevent the current situation where automerge is
enabled but not used due to the fact that renovate bots runs as cronjob
instead of being the server-side application.

config.js

@@ -10,6 +10,7 @@ module.exports = {
   onboardingConfig: {
     extends: ['config:base'],
     prConcurrentLimit: 5,
-    updateLockFiles: true
+    updateLockFiles: true,
+    gitLabAutomerge: true
   }
 };