Resolve "Markdown is not rendering, "Error loading viewer": undefined method `html_escape'"

What does this MR do?

Makes calls to html_escape into ERB::Util.html_escape

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

Why was this MR needed?

In a customer reported instance, they were getting an "Error loading viewer": undefined method 'html_escape' during markdown rendering. The call to html_escape was ambiguous, so we now made it explicit.

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?

Closes #45436 (closed)