Premium plans are eligible for combined trial

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/497814

What does this MR do and why?

Allows premium plans to register for the combined Ultimate + Duo Enterprise trial

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #497814