Administration: Include the new plugin and theme versions in email notifications.

This helps administrators keep track of which versions of plugins and themes are running on their site when auto-updates occur.

Props audrasjb, pbiron.
Fixes #50350.
Built from https://develop.svn.wordpress.org/trunk@48517


git-svn-id: https://core.svn.wordpress.org/trunk@48279 1a063a9b-81f0-0310-95a4-ce76da25c4cd