Ensure that cached_markdown_version is handled correctly for CommonMark

What does this MR do?

When a cached markdown field is saved, it updates the cached_markdown_version field to the latest value. As we don't want RedCarpet rendered fields to be upgraded to CommonMark fields, add the logic to upgrade the version to only the latest RedCarpet or CommonMark version.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

See above

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [-] Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/43011