Accessibility: the iframes used for the update progress in the Updates screen...

Accessibility: the iframes used for the update progress in the Updates screen need title attributes to describe their content.

Fixes #34763.
Built from https://develop.svn.wordpress.org/trunk@35854


git-svn-id: https://core.svn.wordpress.org/trunk@35818 1a063a9b-81f0-0310-95a4-ce76da25c4cd