Update markdownlint and Vale for project

What does this Merge Request do and why?

Make use of new linting image from: gitlab-docs!2422 (merged).

CC @toon @ashmckenzie

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.