Upgrade/Install: Don't trigger database upgrade on Ajax requests via `wp-admin/async-upload.php`.

Props schlessera, jgrodel, elrae, davidbaumwald, hareesh-pillai, adamsilverstein, SergeyBiryukov.
Fixes #39459.
Built from https://develop.svn.wordpress.org/trunk@48565


git-svn-id: https://core.svn.wordpress.org/trunk@48327 1a063a9b-81f0-0310-95a4-ce76da25c4cd