Fix border bottom not showing on blocks in sidebar

What does this MR do and why?

Fixes the bottom of the blocks in the sidebar on issues and merge requests not showing (does not apply to the feature flagged moved_mr_sidebar sidebar)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Issues & epics

• Look at an issue or epic

Merge requests

• Horizontal lines should only show up on old version of MR sidebar

1. Disable :moved_mr_sidebar and verify lines are in sidebar
2. Enable :moved_mr_sidebar and verify lines are not in sidebar (we will likely add some sort of separation here at some point in the future before releasing to everybody)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.