ci: annotate nodejs version for renovate upgrades

I noticed in passing that the nodejs version isn't annotated for renovate upgrades.

This means that we won't automatically upgrade, potentially missing security vulnerability updates, etc.

Documentation on these annotations are available at https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/renovate-bot.md#annotating-tool-versions-and-gitlab-ci-other-versionsyml-to-support-renovate