Trial onboarding UX enhancements

What does this MR do?

Trial onboarding UX enhancements

• Create your first group step: The form container is bigger than on the other pages. We should fix the styling class.
• Create your first project step: It seems that we're missing the trial confirmation banner on production. This banner is already here in this flow. I think it's a matter of making sure it's triggered at this stage.
• Getting started with GitLab step: The background of the page is gray instead of white which causes contrast issues with the illustration. This should be a quick fix.
• The box size for the whole process is now max 460px instead of a mix of 400px and that value.

Related to #296507 (closed) and behind a feature flag for an experiment that isn't yet launched #276704 (closed)

Screenshots

https://www.youtube.com/watch?v=U6O4Gxkj-x0

• most changes are for the gray background turning into white, which isn't noticeable

Before	After
No trial activation banner

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