Set Lists

You can use particular single rules, or whole list of rules, called "set lists":

use Rector\Config\RectorConfig;
use Rector\Set\ValueObject\SetList;

return static function (RectorConfig $rectorConfig): void {
    $rectorConfig->sets([SetList::DEAD_CODE]);

    $rectorConfig->paths([__DIR__ . '/src']);
};

That way you can include group of rules that focus on certain topic, e.g. in this case on dead detection. It makes config small and clear.

Level Sets

How can you upgrade to the PHP 7.3?

use Rector\Config\RectorConfig;
use Rector\Set\ValueObject\SetList;

return static function (RectorConfig $rectorConfig): void {
    $rectorConfig->sets([SetList::PHP_73]);

    $rectorConfig->paths([__DIR__ . '/src']);
};

That's one of way, but what about all previous PHP versions like PHP 7.2, 7.1, 7.0 etc.?

Instead of listing all the PHP version sets, you can use "level sets":

use Rector\Config\RectorConfig;
use Rector\Set\ValueObject\LevelSetList;

return static function (RectorConfig $rectorConfig): void {
    $rectorConfig->sets([LevelSetList::UP_TO_PHP_73]);

    $rectorConfig->paths([__DIR__ . '/src']);
};

That way you can use all the sets that are needed to upgrade your code to the desired PHP version in single line.