Prettify billing plans section

What does this MR do?

Prettifies the billing plans a bit:

1. Cards now correctly work with free (we still use this on CustomersDot) and default plans.
2. Adjusted responsiveness for cards.
3. Uses utility classes from @gitlab/ui.

Screenshots (strongly suggested)

Before	After

NOTE: Buttons Contact sales are visible on the left image because the their feature flag is enabled. These buttons are not affected by this merge request.

Responsiveness in action (video)

Before

billing_plans_before

After

billing_plans_after

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team