Move back to a gitlab_kramdown version that doesn't require Ruby 2.7.6
To avoid: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/2628392180, downgrade version of gitlab_kramdown
we use.
Edited by Evan Read
To avoid: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/2628392180, downgrade version of gitlab_kramdown
we use.