fix(ras-acc): newspack ui heading font weights

[chickenn00dle]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1207817176293825/1207878733150043/f

Seems all bolded text, headings included, should have a font weight of 600 per the task above. This PR forces the 600 font weight on headings.

How to test the changes in this Pull Request:

1. Open the checkout modal and confirm each heading has a font weight of 600 throughout the modal
2. Repeat the above step for the auth modal

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[laurelfulford]

Looks good!

I did the functional testing for this first, so seeing the actual code change under the hood made me laugh 😄