Onboarding Improvements: make text larger on Quick Start prompt

[momo-ozawa]

Ref: p5T066-2Rr-p2#comment-10831

Description

This PR makes the text on the Quick Start prompt larger, per suggestion from Calls for Testing WPiOS 18.8

To test

1. Do a fresh install of WPiOS
2. Login
3. Choose a site
4. ✅ Notice "Want a little help..." and "Learn the basics..." is in a larger font size

Before	After

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.