Adjust legacy onboarding experiment

What does this MR do and why?

The experiment controls which onboarding path new trial users see when they sign up - either a "Get Started" page or a "Learn GitLab" page.

Adds an only_assigned parameter to ensure the experiment only runs for users who have been specifically assigned to it on the sidebar panel.

All the underlying functionality remains the same - this is purely a naming change to better describe what the experiment does.

References

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How to set up and validate locally

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Related to #572457