Upgrade/Install: Simplify compatibility checks for uploaded plugins and themes...

Upgrade/Install: Simplify compatibility checks for uploaded plugins and themes for better readability.

Use `$new_plugin_data` and `$new_theme_data` as a shorthand for the corresponding `$this->upgrader` properties.

Follow-up to [48390], [48448].

Props afragen.
See #9757.
Built from https://develop.svn.wordpress.org/trunk@48455


git-svn-id: https://core.svn.wordpress.org/trunk@48224 1a063a9b-81f0-0310-95a4-ce76da25c4cd