Rule source on GitHub

DowngradeReadonlyClassRector

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

-final readonly class SomeClass
+final class SomeClass
 {
-    public string $foo;
+    public readonly string $foo;

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

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp82\Rector\Class_\DowngradeReadonlyClassRector;

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