This diff cleans up the relationship tab in the thread settings modal to follow the conventions of the new modal api. Subsequent diffs will handle the other tabs in the thread settings modal.
The relationship tab actually does not have a primary button; however, if we don't pass in a placeholder primary button, the thread settings modal will change height + position, so I added a placeholder div so the following does not happen
Linear task: https://linear.app/comm/issue/ENG-5943/extendmodify-the-modal-props-api-to-follow-new-modal-designs
Depends on D11032