Fix labels tooltip in collapsed Epics sidebar

What does this MR do?

This MR adds missing data-boundary="viewport" to target element which shows tooltip for Labels in Epics collapsed sidebar.

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
• ~~[ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• [ ] EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• [ ] Security reports checked/validated by reviewer

Closes #8703 (closed)