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).
Merge Request checklist
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This change is backward compatible. If not, please include steps to communicate to our users. 
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Tests added for new functionality. If not, please raise Issue to follow-up. 
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Documentation added/updated, if needed. 
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gdk doctortest added, if needed.
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Add the ~highlightlabel if this MR should be included in theCHANGELOG.md.