Administration: Bring the update nag in line with other admin notices.

In addition to more consistent display, this gives it a visible border on the About page.

Props desrosj, nikhilbhansi, afercia, SergeyBiryukov.
Fixes #50372.
Built from https://develop.svn.wordpress.org/trunk@48316


git-svn-id: https://core.svn.wordpress.org/trunk@48085 1a063a9b-81f0-0310-95a4-ce76da25c4cd