Fully hook up functionality of new Experience Level sign-up step

What does this MR do?

This MR taps into work done in !32145 (merged) and completes the sign-up flow for the New Sign-ups Onboarding Issues Board experiment.

It furthers the work started in !31415 (merged) & continued in !32784 (merged).

No changelog is needed since this new page and the new sign-up flow it is a part of are all behind an experiment feature flag.

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

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Addresses #220110 (closed) Relates to gitlab-org/growth/engineering#5363 (closed)