Update rdoc to 6.0.4

What does this MR do?

Updates rdoc from 4.2.2 to 6.0.4.

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

Upstream diff: https://github.com/ruby/rdoc/compare/v4.2.2...v6.0.4

Why was this MR needed?

To keep dependency up to date.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests passed for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?

Closes #46849 (closed)