Modals within danger zone of repository settings do not focus trap

[JoshMK]

Select Topic Area

Bug

GitHub Feature Area

Repositories

Body

When selecting "Archive this repository" or "Delete this repository" within a repository's settings, confirmation modals are opened.

Once either modal has opened, pressing Tab or Shift plus Tab can navigate the user outside the modal and into the browser's address bar.

Expected behavior: Shift and Shift plus Tab should restrict the user's focus to the close modal button and focusable content (inputs, etc.) within the modals.

Actual behavior: pressing Shift or Shift plus Tab in sequence will move focus outside the modals and into the browser.

[github-actions[bot]]

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