v6: Remove extra bottom border on card navigation nav-tabs (#42571)

v6: Remove extra bottom border on card navigation nav-tabs (#42571)

* v6: Remove extra bottom border on card navigation nav-tabs

Adding navigation to a card’s header with the current v6 adds an extra border which is visible in the documentation, see https://v6-dev--twbs-bootstrap.netlify.app/docs/6.0/components/card/#navigation

<img width="1834" height="766" alt="Image" src="https://github.com/user-attachments/assets/2b87d10f-1cf5-4a74-a2df-047a55ae0fb9" />

This commit removes the extra border.

* Update scss/_card.scss

Co-authored-by: Mark Otto <markdotto@gmail.com>
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Co-authored-by: Mark Otto <markdotto@gmail.com>