Update Markdownlint

[MSevey]

Our current action is this: https://github.com/igorshubovych/markdownlint-cli

Which under the hood targets this: https://github.com/DavidAnson/markdownlint

And the same author has cli and actions.
https://github.com/DavidAnson/markdownlint-cli2
https://github.com/DavidAnson/markdownlint-cli2-action