Add weight to issue hook

What does this MR do?

Add weight field to the hook payload.

Why was this MR needed?

So the weight is available when an issue is updated, without having to go back to the issue API.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #6274 (closed) - Weight is not included in the issue hook