I18N: Use `wp.i18n` for translatable strings in `wp-admin/js/set-post-thumbnail.js`.

This removes the usage of `wp_localize_script()` for passing translations to the script and instead adds the translatable strings in the script directly through the use of `wp.i18n` and its utilities.

Props swissspidy, ocean90.
See #20491.
Fixes #50605.
Built from https://develop.svn.wordpress.org/trunk@48396


git-svn-id: https://core.svn.wordpress.org/trunk@48165 1a063a9b-81f0-0310-95a4-ce76da25c4cd