Filters whether to notify the site administrator of a new core update.

By default, administrators are notified when the update offer received
from WordPress.org sets a particular flag. This allows some discretion
in if and when to notify.

This filter is only evaluated once per release. If the same email address
was already notified of the same new version, WordPress won’t repeatedly
email the administrator.

This filter is also used on about.php to check if a plugin has disabled
these notifications.

if ( ! apply_filters( 'send_core_update_notification_email', $notify, $item ) ) {