[Site Design Revamp] Update title text and remove subtext

[twstokes]

Fixes items B and C in #18433.

This PR:

• Renames the title from "Choose a design" to "Choose a theme"
• Removes the subtext from the view (it was decided to not repurpose this text for a11y)

Before	After

To test:

1. Start the Site Creation flow and navigate to the Site Design screen
2. Expect that the title is "Choose a theme"
3. Expect there to be no subtext (e.g. "Pick your favorite homepage layout...")

⚠️ On screen rotation there is a known issue that wasn't introduced with this PR.

Regression Notes

1. Potential unintended areas of impact

   • Visual impacts of views that rely on the Collapsable Header View
     • Page layout view (when creating a new page)
     • Site Intent view (when creating a site)
     • Domain selection view (when creating a site, if not prompted to enter the site name)
     • Blogging prompts feature introduction view (this appears to be disabled behind a Feature Flag)

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

   • Visiting the views above and testing scrolling + orientation changes.

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

   • These changes are primarily visual

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.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr18474-3e9f2d5 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr18474-3e9f2d5 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[peril-wordpress-mobile]

	Warnings
⚠️	PR is not assigned to a milestone.

Generated by 🚫 dangerJS

[ovitrif]

Works as expected and there's no regression.

Great work @twstokes 🚀

[twstokes]

Thank you @ovitrif!