Use wp.i18n for localization in multi-post-thumbnails-admin.js

[julia-smith]

What does this PR do?

Uses wp.i18n for localization in admin metaboxes because setPostThumbnailL10n was removed from WordPress 5.5.

Why are we doing this?

The plugin began producing JavaScript errors with the 5.5 update.

References

• https://make.wordpress.org/core/2020/08/05/more-support-for-javascript-i18n-in-wordpress-5-5/
• https://make.wordpress.org/core/2018/11/09/new-javascript-i18n-support-in-wordpress/
• https://developer.wordpress.org/block-editor/packages/packages-i18n/
• https://core.trac.wordpress.org/ticket/50605

[kasparsd]

This fixes #146.