Suppress admin notices on the about page.

Brings back [19393], accidentally reverted in [26718].

props ocean90.
fixes #26519.

Built from https://develop.svn.wordpress.org/trunk@26847


git-svn-id: https://core.svn.wordpress.org/trunk@26734 1a063a9b-81f0-0310-95a4-ce76da25c4cd