Accessibility: Add the `aria-hidden` attribute to the auto-update related icons.

`<span>` elements used for displaying icons should use the `aria-hidden` attribute as they are purely decorative.

Props audrasjb, ryokuhi.
Fixes #50293.
Built from https://develop.svn.wordpress.org/trunk@47910


git-svn-id: https://core.svn.wordpress.org/trunk@47684 1a063a9b-81f0-0310-95a4-ce76da25c4cd