Security: Improve the wording of messages in `wp_get_auto_update_message()`.

Props burhandodhy, johnbillion, apedog.
Fixes #50489.
Built from https://develop.svn.wordpress.org/trunk@48202


git-svn-id: https://core.svn.wordpress.org/trunk@47971 1a063a9b-81f0-0310-95a4-ce76da25c4cd