Ignore task item checkboxes in HTML commented sections

What does this MR do and why?

When markdown tasks are commented out with an HTML comment, they were still being counted in tasks completed status.

Now we ignore task items that are in HTML block comments and in code blocks

How to set up and validate locally

Put the following markdown in an issue description.

Any text before the list

- [ ] First item

<!-- a comment
- [ ] Item in comment, ignore
 rest of comment -->

- [x] Second item
* [x] First item

```
- [ ] Item in code, ignore
```

* [ ] Second item

You should see 2 of 4 checklist items completed at the top of the page. You may have to refresh the screen first.

Verify that checking the checkboxes work correctly.

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 #270947 (closed)