Remove mdv from workflow

[ralfhandl]

Suggested improvements to

• Improve Markdown linting and use it on more files OAI/OpenAPI-Specification#4515

Changes:

• workflow validate-markdown.yaml: remove mdv, align with npm script validate-markdown
• package.json: add dev dependency to markdownlint-cli2 because we depend on it and this is the place to document that
• refresh package-lock.json

[lornajane]

Thanks for amending the workflow!