Rule source on GitHub

DowngradeNewInInitializerRector

Replace New in initializers

 class SomeClass
 {
     public function __construct(
-        private Logger $logger = new NullLogger,
+        private ?Logger $logger = null,
     ) {
+        $this->logger = $logger ?? new NullLogger;
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\FunctionLike\DowngradeNewInInitializerRector;

return RectorConfig::configure()
    ->withRules([
        DowngradeNewInInitializerRector::class,
    ]);