ConfirmationDialog header does not match Dialog’s

### Description

By overriding the default `renderHeader` function, some default styling of the Dialog gets stripped (header divider).

### Steps to reproduce

1. See https://primer.style/product/components/confirmation-dialog/ (no border under header)
2. See https://primer.style/product/components/dialog/ (header has a divider)

### Version

v35.2.0

### Browser

_No response_