Improve formatting of PR links added to issues #517

This is a follow-up of:

• Add PR links to the issue #501
• Add Pull Request Link to Issue #511

Currently the code generated when multiple PRs are linked to an issue looks like:

<!-- gh-pr-number: gh-98920 -->
* PR: gh-98920
<!-- /gh-pr-number -->


<!-- gh-pr-number: gh-98952 -->
* PR: gh-98952
<!-- /gh-pr-number -->


<!-- gh-pr-number: gh-98969 -->
* PR: gh-98969
<!-- /gh-pr-number -->

And the output looks like:

It would be better to:

1. combine these into a single multi-item list, rather than multiple single-item lists
2. use a checklist or tasklist instead of a regular bullet list

If the PRs are combined in a single list, we won't be able to just append a new PR to the end, but we will need to add the PR link before the closing tag (<!-- /gh-pr-number -->). In addition, the opening tag lists the PR number, so that will need to be updated too -- unless we remove the number from the tag, since it's already included in the list.

Using tasklists is probably better than checklists, and for that, the generated source should look like:

<!-- gh-linked-prs -->
``` [tasklist]
### Linked PRs
- [ ] gh-98920
- [ ] gh-98952
- [ ] gh-98969
```
<!-- /gh-linked-prs -->

(there should be no space between ``` and [tasklist], I added it here otherwise the output breaks).

Since tasklists support a title (e.g. ### Linked PRs), there is no need to repeat PR: before each PR number. (Note that tasklists are a beta feature enabled for the python org, and don't work for other orgs (yet).)

(cc @saadmk11)