Admin custom configuration form add more field issue

I have created below admin custom configuration form by taking a reference of below examples:

1. https://git.drupalcode.org/project/examples/-/blob/8.x-1.x/form_api_example/src/Form/AjaxAddMore.php
2. How do I add a "Add more" button?

My custom form is rendered correctly on first time load but when i click on "Add Product" button nothing happen. As per example the form should rebuild again and prepend the fields. I assume here it could be possible due to something deprecated in Drupal 9 as all above examples are working fine in Drupal 8.

How can I correctly add the Add more button?

<?php

namespace Drupal\commerce_product_quantity\Form;

use Drupal;
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Configure commerce_product_quantity settings for this site.
 */
class SettingsForm extends ConfigFormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId(): string
  {
    return 'commerce_product_quantity_settings';
  }

  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames(): array
  {
    return ['commerce_product_quantity.settings'];
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state): array
  {
    /* Fetch All Products */
    $result = Drupal::entityQuery('commerce_product')
      ->execute();
    $titles = array('none' => "- Select -");

    foreach ($result as $product_id) {
      $entity_manager = \Drupal::entityTypeManager();
      $product = $entity_manager->getStorage('commerce_product')->load($product_id);
      $titles[$product->product_id->value] = $product->get('title')->value;
    }

    $field_count = $form_state->get('fields_count');
    $form['#tree'] = TRUE;
    $form['product_quantity_fieldset'] = [
      '#type' => 'fieldset',
      '#title' => $this->t('Products With Total Allowed Quantity'),
      '#attributes' => ['id' => 'product-fieldset-wrapper'],
    ];

    if (empty($field_count)) {
      $form_state->set('fields_count', 1);
      $field_count = 1;
    }

    for ($i = 0; $i < $field_count; $i++) {
      $form['product_quantity_fieldset']['product'.$i] = [
        '#type' => 'select',
        '#title' => t('Add Product'),
        '#options' => $titles,
      ];

      $configName = "commerce_product_quantity.settings.".$i;
      $form['product_quantity_fieldset']['quantity'.$i] = [
        '#type' => 'number',
        '#title' => $this->t('Quantity'),
        '#default_value' => $this->config($configName)->get('quantity'),
      ];
    }
    $form['actions'] = [
      '#type' => 'actions',
    ];
    $form['product_quantity_fieldset']['actions']['add_name'] = [
      '#type' => 'submit',
      '#value' => $this->t('Add Product'),
      '#submit' => array('::addOne'),
      '#ajax' => [
        'callback' => '::addmoreCallback',
        'wrapper' => 'product-fieldset-wrapper',
      ],
    ];
    if ($field_count > 1) {
      $form['product_quantity_fieldset']['actions']['remove_name'] = [
        '#type' => 'submit',
        '#value' => $this->t('Remove one'),
        '#submit' => array('::removeOne'),
        '#ajax' => [
          'callback' => '::removeCallback',
          'wrapper' => 'product-fieldset-wrapper',
        ]
      ];
    }
    $form_state->setCached(FALSE);
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];

    return $form;
  }

  public function addOne(array &$form, FormStateInterface $form_state) {
    $field_count = $form_state->get('fields_count');
    $add_button = $field_count + 1;

    \Drupal::messenger()->addStatus($add_button);
    $form_state->set('fields_count', $add_button);
    $form_state->setRebuild();
  }

  public function removeOne(array &$form, FormStateInterface $form_state) {
    $field_count = $form_state->get('num_names');
    $add_button = $field_count + 1;
    $form_state->set('num_names', $add_button);
    $form_state->setRebuild(TRUE);
  }

  public function addmoreCallback(array &$form, FormStateInterface $form_state) {
    $add_button = $form_state->get('fields_count');
    return $form['product_quantity_fieldset'];
  }

  public function removeCallback(array &$form, FormStateInterface $form_state) {
    $field_count = $form_state->get('fields_count');
    if ($field_count > 1) {
      $remove_button = $field_count - 1;
      $form_state->set('fields_count', $remove_button);
    }
    $form_state->setRebuild(TRUE);
  }

  public function validateForm(array &$form, FormStateInterface $form_state) {

  }

  public function submitForm(array &$form, FormStateInterface $form_state) {
   
  }
}