fix border-radius for related issues

What does this MR do?

This MR tries to fix the border-radius issues for related-issues section. The border is not fully drawn, because the content expands into the border-radius area. The overflow property forces the content to stay inside the borders and therefor renders the border without interruption.

What are the relevant issue numbers?

Closes #8963 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• 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