Rule source on GitHub

DowngradeReadonlyPropertyRector

Remove "readonly" property type, add a "@readonly" tag instead

 class SomeClass
 {
-    public readonly string $foo;
+    /**
+     * @readonly
+     */
+    public string $foo;

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

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp81\Rector\Property\DowngradeReadonlyPropertyRector;

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