Add "free upgrade" marketing UX

What does this MR do?

There is an envisioned scenario that in the future GitLab would provide free upgrades to certain plans.

This MR adds the UX to the billings page for this flow.

Parent issue: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2410

Designs: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2131/designs/SaaS__Billing_page_free_upgrade.png

Things that differ from the designs but were discussed and accepted

• We removed the Contact Sales button from all offer-eligible cards since we want to send the users through self-service.

Screenshots

Screenshots are here: internal slack thread

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