docs: update node version for gitlab ci config (#2592)

* Update deploy.md

Here is my issue: https://github.com/vuejs/vuepress/issues/2584
I've found that node:9.11.1 is too old and in the `Prerequisites` part of the guide, it's recommend to use node 10+ so update a small comment could be useful. I don't know if it is applicable to delete `node:9.11.1` and replace with `node:14.8.0` so I just add a comment

* Update deploy.md

packages/docs/docs/guide/deploy.md

@@ -101,8 +101,7 @@ deploy:
 3. Create a file called `.gitlab-ci.yml` in the root of your project with the content below. This will build and deploy your site whenever you make changes to your content:
 
 ``` yaml
-image: node:9.11.1
-
+image: node:10.22.0
 pages:
   cache:
     paths: