Fix navigation buttons not scaling with large fonts in Site Intent

[Gio2018]

Fixes #18230

To test:

• Checkout this branch
• Enable site name, in order to test the full navigation: this can be done by overriding

private var shouldShowSiteName: Bool {
        return nameVariant == .treatment(nil) && FeatureFlag.siteName.enabled
}

in SiteCreationWizardLauncher and temporarily set it to true

• Build/run and start the site creation flow
• make sure the appearance of the navigation bar and button looks correct in all the navigation steps
• Go to the device settings and change the font size in Accessibility
• Restart the site creation flow, and verify that the Cancel and Skip buttons scale correctly
• Check that the navigation bar looks correct in the rest of the flow

Regression Notes

1. Potential unintended areas of impact
   Low, this change only affects the appearance of the navigation bar in the site creation flow.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Test the entire site creation flow

3. What automated tests I added (or what prevented me from doing so)
   none
   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 pr18559-63bb2db 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 pr18559-63bb2db on your iPhone

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

[twstokes]

LGTM @Gio2018! 🚀