Style issues with confirmation dialog

[pmarsceill]

Describe the bug
Some styling issues with confirmation dialog component.

To Reproduce
Steps to reproduce the behavior:
Look at this component.

1. Title text is too bold --
2. Border radius on bottom (buttons) is too small compared to top border radii

Expected behavior

1. Should use font-weight: 600 for bold text as the rest of the bold text in Primer does
2. Border radii are consistent in all 4 corners

Screenshots

Desktop (please complete the following information):
All

Smartphone (please complete the following information):
n/a

Additional context
n/a