WIP: Checkbox in issue description cannot be checked if the description contains a blockquote

What does this MR do?

We use the CommonMark SOURCEPOS flag to map lines in the source markdown to the rendered HTML. We then use that to determine which checkbox has been clicked and which line of source it belongs to.

However, when using the GitLab specific blockquote syntax,

>>>
some quoted text
>>>

the line numbers no longer match. This is because we add > in front of the quoted text, and remove the two marker lines. Which changes the number of lines of source. Since that is what is run through CommonMark, the line numbers are generated against that source, not the original user source. Making the line number incorrect.

This MR goes back to using the number of the checkbox in the DOM to determine which checkbox has been checked.

What are the relevant issue numbers?

Closes #58717 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated via this MR
• [ ] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• Security reports checked/validated by reviewer