Remove background color from card-body style

What does this MR do?

Removes background color from card-body style.

Are there points in the code the reviewer needs to double check?

The problem was that card-body had a background-color in app/assets/stylesheets/framework/panels.scss which was affecting a lot of cards inside /admin. I was planning to append the following which would affect all cards across the platform:

border-bottom-left-radius: 0.20rem;
border-bottom-right-radius: 0.20rem;

But instead, removing that background-color and using text and background utilities to change a few card styles seemed like a better approach, since there're only a few occurrences where it's needed to have a different border or header color other than the default. What do you think?

Screenshots (if relevant)

Here're some of the panels affected.

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer

What are the relevant issue numbers?

Closes #49350 (closed)