Prevent registration group creation changing registration_type

What does this MR do and why?

Prevent registration group creation changing registration_type

• currently during group creation during registration also has the side effect of changing the registration_type to 'invite' due to the group.add_owner that travels through the invite registration conversion as we wanted to properly convert users to an invite if they were invited by another user to a different group or project and were still in onboarding.
• To fix this so that the Groups::CreateService doesn't errantly change the registration type, we'll handle the invite conversion at the service layer that is only hit during UI/API calls instead. I believe this is a more targeted approach and cover all use cases we care about still and still cover the 2fa case for finishing onboarding as well.
• see !149318 (diffs, comment 1858265158) for initial introduction/why invite conversion of the registration_type was done.
• note: I took 2 different approaches to this issue which can be seen in the different commits on this MR. In the end, I think the current approach is best/more targeted.

Changelog: fixed EE: true

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Related to #485500 (closed)