Update gitlab-docs to Ruby 3

This is one of the projects which requires Ruby to be updated: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.ruby-version

docs-lint links in GitLab uses the image from https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitlab-ci.yml#L241.

It seems that it shouldn't block our Ruby 3 transition for GitLab (we may still use the docs-lint image with 2.7).
If that is so, we could update gitlab-docs later (after having a CI pipeline with Ruby 3 in GitLab).