Use Drupal Field API instead of raw SQL query

In order to let Drupal core to proper caching on container level as well as handle transactions without race conditions, we should follow best practices and use Drupal API to pull data from the field, not a raw SQL query.
Drupal API also handles different DB backends properly, while raw SQL queries would have issues.

Before

            $query = \Drupal::database()->select('node__field_category', 'n');
            $query->leftjoin('taxonomy_term_field_data', 'tfd', 'tfd.tid = n.field_category_target_id');
            $query->addExpression('GROUP_CONCAT(DISTINCT tfd.name)', 'cats');
            $query->condition('n.entity_id', $nid);
            $categories = $query->execute()->fetchField();

After

            $node = \Drupal::routeMatch()->getParameter('node');
            if ($node instanceof NodeInterface) {
              if (!empty($node->get('field_category'))) {
                $categories = $node->get('field_category');
              }
            }