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Configurable

ReplaceFieldgroupToFieldsetRector

Replace '#type' => 'fieldgroup' with '#type' => 'fieldset' in render arrays

-$form['account'] = ['#type' => 'fieldgroup', '#title' => $this->t('Account settings')];
+$form['account'] = ['#type' => 'fieldset', '#title' => $this->t('Account settings')];

Configure your rector.php:

<?php

use DrupalRector\Drupal11\Rector\Deprecation\ReplaceFieldgroupToFieldsetRector;
use DrupalRector\Rector\ValueObject\DrupalIntroducedVersionConfiguration;
use Rector\Config\RectorConfig;

return RectorConfig::configure()
    ->withConfiguredRule(ReplaceFieldgroupToFieldsetRector::class, [
        new DrupalIntroducedVersionConfiguration('11.2.0'),
    ]);