Allow new joining_project in the onboarding_status jsonb column

What does this MR do and why?

Allow new joining_project in the onboarding_status jsonb column

• we must add them first here due to our setting of "additionalProperties": false before using and deploy it out to the entire GitLab.com environment. If we do not deploy this before using we could get into a case where we store a value on canary and then try to save a user update on non-canary and cause an error on user validation. see #461404 (closed) for more info on it.
• see #498743 (closed)

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Related to #498743 (closed)