Fix border on description box

What does this MR do and why?

Fix border disappearing on description box in new epics and when editing existing issues

• Fixes regression caused by !117734 (merged) and !119156 (merged)
• Fixes #410665 (closed)

Screenshots or screen recordings

Scenario	Before	After
Create a new epic
Edit an existing issue description. Feature flag :content_editor_on_issues off.

How to set up and validate locally

Steps outlined in the scenario descriptions above.

MR acceptance checklist

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