Forms: use 'Stack' component from wp-ui in wp-build dashboard (#46680)

* Forms: use Stack from wp-ui

* changelog

* Remove unused direction and className

* Replace span with Stack

* Replace more spans

* Recover tabs styling

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/21195786249

Upstream-Ref: Automattic/jetpack@b439743

Author: chihsuan