I18N: Move the `aria-label` text in `get_theme_update_available()` to a...

I18N: Move the `aria-label` text in `get_theme_update_available()` to a separate string for easier translation.

Add translator comments.

Props ramiy.
Fixes #36048.
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git-svn-id: https://core.svn.wordpress.org/trunk@36812 1a063a9b-81f0-0310-95a4-ce76da25c4cd