So I did a standard composer upgrade.

sudo -u www-data /usr/bin/php8.4 /usr/local/bin/composer update "drupal/*" --with-all-dependencies

and all went well.
Then I opened my browser at example.com/update.php and got a 500.

AH01071: Got error 'PHP message: PHP Warning: require_once(autoload_runtime.php): Failed to open stream: No such file or directory in /var/www/website.com/web/update.php on line 13; PHP message: PHP Fatal error: Uncaught Error: Failed opening required 'autoload_runtime.php' (include_path='.:/usr/share/php') in /var/www/website.com/web/update.php:13\nStack trace:\n#0 {main}\n thrown in /var/www/website.com/web/update.php on line 13'

So in the update.php and the index.php I had to change this bit:

// require_once 'autoload_runtime.php';
require_once __DIR__ . '/../vendor/autoload_runtime.php';

What's wrong on my system? I can't imagine this should need to be done?