Accessibility: Correctly associate the search input label in the new Themes...

Accessibility: Correctly associate the search input label in the new Themes favorites tab introduced in [35527].

Fixes #35122.
Built from https://develop.svn.wordpress.org/trunk@35977


git-svn-id: https://core.svn.wordpress.org/trunk@35942 1a063a9b-81f0-0310-95a4-ce76da25c4cd