Fix Vue datatype errors for markdownVersion parsing

What does this MR do?

NotesApp requires markdownVersion prop as number, but same prop is not parsed into number correctly, leading to datatype errors from Vue while using NoteApp on Epics (EE feature). This was missed out from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19672 is currently affecting EE master.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Conforms to the code review guidelines
  • [ ] Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [ ] Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [ ] Internationalization required/considered
• [ ] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#43011 (closed)