bin/magento error after update

The issue can be fixed by manually editing composer.json autoload section to

"autoload": {
    "psr-4": {
        "Magento\\Framework\\": "lib/internal/Magento/Framework/",
        "Magento\\Setup\\": "setup/src/Magento/Setup/",
        "Magento\\": "app/code/Magento/",
        "Zend\\Mvc\\Controller\\": "setup/src/Zend/Mvc/Controller/"
    }
}

and then run

composer dumpautoload
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy

It will work.

Following the upgrade instructions here https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html should take care of this issue. In particular for this error make sure to update your autoload section.