code: public function run(): void
      {
          $statement = $this->connection->query("
              SELECT
                  id, framework
              FROM
                  tl_layout
              WHERE
                  framework != ''
          ");
  
          while (false !== ($layout = $statement->fetch(\PDO::FETCH_OBJ))) {
              $framework = '';
              $tmp = StringUtil::deserialize($layout->framework);
  
              if (!empty($tmp) && \is_array($tmp)) {
                  if (false !== ($key = array_search('layout.css', $tmp, true))) {
                      array_insert($tmp, $key + 1, 'responsive.css');
                  }
  
                  $framework = serialize(array_values(array_unique($tmp)));
              }
  
              $stmt = $this->connection->prepare('
                  UPDATE
                      tl_layout
                  SET
                      framework = :framework
                  WHERE
                      id = :id
              ');
  
              $stmt->execute([':framework' => $framework, ':id' => $layout->id]);
          }
  
          // Add the "viewport" field (triggers the version 3.3 update)
          $this->connection->query("
              ALTER TABLE
                  tl_layout
              ADD
                  viewport varchar(255) NOT NULL default ''
          ");
      }
docs: {@inheritdoc}