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ProtectedStaticModulesPropertyRector

"public static $modules" will have its visibility changed to protected.

 class SomeClassTest {
-  public static $modules = [];
+  protected static $modules = [];
 }

Configure your rector.php:

<?php

use DrupalRector\Drupal9\Rector\Property\ProtectedStaticModulesPropertyRector;
use Rector\Config\RectorConfig;

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