Add merge request count to each issue on issues list
What does this MR do?
Adds merge requests closing issuable to issues list.
Screenshots (if relevant)
Does this MR meet the acceptance criteria?
- Changelog entry added
- Documentation created/updated
- API support added
- Added for this feature/bug
- All builds are passing
- Conform by the merge request performance guides
- Conform by the style guides
Branch has no merge conflicts with
master(if it does - rebase it please)
- Squashed related commits together
What are the relevant issue numbers?
Closes #19786 (closed)