Cannot add label to issue from board

What does this MR do?

When a group has several projects, each with different labels, editing the labels of an issue in the group board would show all labels from all projects, even if a label was not valid for that issue.

We now query the labels for the issue's project (which includes the project and any group labels).

What are the relevant issue numbers?

Closes #51620 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides