Stepper: Fix accessibility issues

[escapemanuele]

I ran a Lighthouse test on /setup/onboarding and saw that there is room for improvement.

Comparing it with Start, I saw that Stepper has a lower accessibility score. We should fix this.

[github-actions[bot]]

OpenAI suggested the following labels for this issue:

• [Feature Group] Site Performance: The accessibility issue pointed out in the Lighthouse test relates directly to the performance and usability of the site.
• [Feature] Site Goals & Onboarding Paths: Improving accessibility will enhance the onboarding experience for new users navigating through the site.
• [Feature] SEO Tools: Accessibility improvements often correlate with better SEO practices, which could be relevant given the context of the issue.
• [Feature] Site Navigation: Addressing accessibility scores can improve how users navigate through the Stepper component.