Cleanup onboarding issues experiment [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Part of the default to onboarding issue board cleanup.

The onboarding issues experiment added the following features:

• It adds Group and Project creation to the signup flow
• It adds an Experience Level picker to the signup flow
• It creates a learn-gitlab project during the signup flow for the user

While the experiment was successful and rolled out to 100% of gitlab.com users for a while, we want to be sure and roll it out behind a normal feature flag that is on by default. However, we don't want to roll this feature out on self managed yet. Therefore, this is gitlab.com only.

Screenshots (strongly suggested)

New onboarding Flow on .com with signup_onboarding enabled:

Signup	Welcome	Group Creation	Project Creation	Experience Level	Group overview

On self managed or when signup_onboarding is disabled:

Signup	Welcome	Dashboard

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Issue: #283933 (closed)