Extend combined_registration productization to FOSS

Summary

We've opted to productize our successful combined_registration experiment where we introduced a combined group and project creation page in the registration flow by starting with productizing to .com and ee. This issue is intended to house conversation and decisioning about whether to extend the productization of that experiment to foss too.

See https://gitlab.com/gitlab-org/gitlab/-/issues/350754#note_1069667616