Frontend: The schema for `.gitlab-ci.yml` is missing support for `!reference`

When using the !reference keyword in a .gitlab-ci.yml, this is shown as invalid syntax.

This affects the web CI validation and VS Code.

I'm assuming the information is sourced from https://gitlab.com/gitlab-org/gitlab/-/raw/master/app/assets/javascripts/editor/schema/ci.json Sadly, I don't know what needs to be adjusted to make this work.