Site Health: Improve the appearance of administration email verification box.

Add `max-width` on desktop view and some padding on smaller screens.

Props chetan200891, azaozz, Clorith, afragen.
Fixes #48607.
Built from https://develop.svn.wordpress.org/trunk@47772


git-svn-id: https://core.svn.wordpress.org/trunk@47548 1a063a9b-81f0-0310-95a4-ce76da25c4cd