Prevent rounded corners in the middle of a BorderBox

[HDinger]

What are you trying to accomplish?

In a Primer::Beta::BorderBox there are constellations in which there are rounded corners somewhere in the middle. For example

render(Primer::Beta::BorderBox.new) do |component|
  component.with_header { "Header" }
  component.with_body { "Body" }
  component.with_row { "Row one" }
  component.with_row { "Row two" }
end

Screenshots

List the issues that this change affects.

Closes #2684

Risk Assessment

• [x] Low risk the change is small, highly observable, and easily rolled back.
• [ ] Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• [ ] High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Merge checklist

• [ ] Added/updated tests
• [ ] Added/updated documentation
• [ ] Added/updated previews (Lookbook)
• [x] Tested in Chrome
• [x] Tested in Firefox
• [x] Tested in Safari
• [x] Tested in Edge

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[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: c3c60f0c839786ebff98e0bf5da48d2a1fabdb51

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Patch

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