Fix class namespaces in Onboarding

[szepeviktor]

All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

\Onboarding was missing from those namespaces.
From #37015 (comment)

[codecov]

Codecov Report

Merging #37056 (447f83c) into trunk (7ec2ad6) will decrease coverage by 0.0%.
The diff coverage is n/a.

❗ Current head 447f83c differs from pull request most recent head 5c7f8fe. Consider uploading reports for the commit 5c7f8fe to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             trunk   #37056     +/-   ##
==========================================
- Coverage     46.7%    46.7%   -0.0%     
  Complexity   17188    17188             
==========================================
  Files          429      429             
  Lines        64820    64820             
==========================================
- Hits         30252    30251      -1     
- Misses       34568    34569      +1     

Impacted Files	Coverage Δ
plugins/woocommerce/includes/class-wc-tax.php	78.6% <0.0%> (-0.2%)	⬇️

[chihsuan]

Hey @szepeviktor Thanks for the fix! Could you please add a changelog via pnpm --filter=woocommerce changelog add?

[szepeviktor]

@chihsuan Changelog added 🍏

[chihsuan]

Looks good. Thanks for fixing it quickly!