Fix SSO subscription registration onboarding dropoffs

The following discussion from !122371 (merged) should be addressed:

• @dstull started a discussion:

  note (non-blocking): Requires code changes to behavior - spawning a new issue from this thread to handle dropoffs on subscription signups we are having when SSO is used.

Current Flow(not sure if it is 100% ideal or not)

• ✅ non SSO subscription registrations follows this
• ❌ SSO subscription registrations do not

I'm skipping the login details and what happens after checkout here as they are not germane to the discussion on what is broken. The entire flow can be read in fairly plain language here

1. Successful sign up
2. Welcome page with subscription wording of Who will be using this GitLab subscription?
3. Checkout
4. Group creation, etc

What is broken with SSO subscription registration?

Steps 2 and 3

• For step 2, we are currently dropping off immediately when we get to the welcome page. At this point we've lost all memory that this is a subscription registration and are now in regular registration onboarding. This is caused by this line of code which causes our memory of being in a subscription to be wiped out.
• For step 3, even if we fixed the issue in step 2, we'd again get that memory wiped if we enabled ensure_onboarding feature flag and drop off into regular registration onboarding at the welcome page. That code is seen here but only becomes an issue once we enable ensure_onboarding feature flag. My assumption here is that regular registration would be affected here too if identity verification was enabled for it, but I believe it is only enabled on SSO right now.

page description	page	what's wrong
welcome page/form		This should say Who will be using this GitLab subscription? instead, indicates we have lost subscription onboarding/dropped off into regular onboarding
company form		we shouldn't even see this page and should be on checkout