Increase the migration welcome screen top inset in iOS 14 only

[salimbraksa]

Fixes #19821

Description

in iOS 15+, the scroll edge appearance of the navigation bar is transparent and becomes translucent when the user starts scrolling. But this behavior is not working in iOS 14, the navigation bar is always translucent. See "Before" screenshot.

Ideally, the navigation bar appearance in iOS 14 should behave like iOS 15+, but it's a little bit challenging to implement that behavior and probably not worth the hassle. This PR simply adds a spacing between the navigation bar and the table view content.

Before	After

Test Instructions

N.B.: Use a device running iOS 14

1. Clean install both WordPress and Jetpack apps
2. In WordPress, log into your account
3. Find any Jetpack Powered feature and tap "Jetpack Powered" badge
4. Tap "Try the new Jetpack app" to trigger the Migration flow
5. The Welcome Migration screen should appear on Jetpack
6. Expect the navigation bar to be translucent and there is a margin between the navigation bar and the table view content.

Regression Notes

1. Potential unintended areas of impact
   None.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   None.

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 pr19822-8234722 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 pr19822-8234722 on your iPhone

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

[dvdchr]

LGTM! Tested and it works as described.