Add SaaS check to onboarding redirect setup

What does this MR do and why?

Add SaaS check to onboarding redirect setup

• our concerns here are growth and marketing related for SaaS only for trials and such which are purely SaaS concepts

Currently the code only saves value to onboarding_step_url if on SaaS and that will keep us from redirecting non-SaaS instances.

However, we shouldn't just rely on that and we should:

• ensure when user signs up that they aren't automatically set onboarding_in_progress to true if not on SaaS
• short circuit checking/any functionality behind the SaaS check

How to set up and validate locally

tests under ee/spec/features/registrations/saas/ covers positive test of this and other tests cover the non-SaaS cases

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #351738 (closed)