Resolve "Duplicate todo is created for already mentioned user"

What does this MR do and why?

Fix the MARKDOWN_HTML_COMMENT_LINE_REGEX so that it uses a non-greedy capture. Otherwise we incorrectly capture this as one comment, rather than two.

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- [ ] two

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How to set up and validate locally

Paste the above markdown into a a fresh issue description. Save it, and refresh the page (in order to get the task number calculated), and you should see 0 of 1 checklist item completed before the Close issue button. When the bug exists, you won't see anything.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393910 (closed)