Posts, Post Types: Change "Featured Image" post label to use sentence case: "Featured image".

This makes it more consistent with other UI elements in the block editor.

Props mcsf.
Fixes #49371.
Built from https://develop.svn.wordpress.org/trunk@47194


git-svn-id: https://core.svn.wordpress.org/trunk@46994 1a063a9b-81f0-0310-95a4-ce76da25c4cd