Rule source on GitHub

DowngradeReadonlyAnonymousClassRector

Remove "readonly" class type on anonymous class, decorate all properties to "readonly"

-new readonly class
+new class
 {
-    public string $foo;
+    public readonly string $foo;

     public function __construct()
     {
         $this->foo = 'foo';
     }
-};
+}

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp83\Rector\Class_\DowngradeReadonlyAnonymousClassRector;

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