Core: Fix onboarding detection in what's new module

[yannbf]

Closes #

What I did

As title says

How to test

I have tested this, but here are the steps to do it yourself:

1. Run a sandbox for template, e.g. yarn task --task sandbox --start-from auto --template react-vite/default-ts
2. Open Storybook in your browser with the following urls:

• http://localhost:6006/ -> it should not fail
• http://localhost:6006/?path=onboarding -> it should not show the whats new notifications
• http://localhost:6006/?path=/story/example-button--primary&onboarding=true -> it should not show the whats new notifications

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[kasperpeulen]

LGTM!