[SetList] Add COMPOSER_BASED constant, remove empty DOCTRINE_BUNDLE_210 set

[TomasVotruba]

Two small DoctrineSetList cleanups.

Add COMPOSER_BASED

config/sets/composer-based.php had no constant, so it could only be referenced by a raw path:

$rectorConfig->sets([__DIR__ . '/../../../config/sets/composer-based.php']);

Now:

use Rector\Doctrine\Set\DoctrineSetList;

return RectorConfig::configure()
    ->withSets([
        DoctrineSetList::COMPOSER_BASED,
    ]);

->withComposerBased(doctrine: true) stays the recommended way - this is for configs that enumerate sets explicitly.

Remove DOCTRINE_BUNDLE_210

config/sets/doctrine-bundle-210.php has been an empty closure since #382 (2025-05), when the AsListener attribute rule moved to the doctrine-bundle 2.8 set:

return static function (RectorConfig $rectorConfig): void {
    // only for BC
};

It is not registered in DoctrineSetProvider, so it never shows up in the prepared set list - the only way to reach it was the deprecated DoctrineSetList::DOCTRINE_BUNDLE_210 constant, which loaded a set that did nothing. File and constant removed together, same as 5f471cf.