The module is always using v2 version for Salesforce auth token request

Problem

The "Salesforce auth token request version" setting in the setting form has the following definition:

// src/Form/MarketingCloudSettings.php

    $form['request_token_version'] = [
      '#type' => 'select',
      '#title' => $this->t('Salesforce auth token request version'),
      '#description' => $this->t('The newer version of token request requires different parameters to the legacy v1.'),
      '#options' => [
        'v1',
        'v2',
      ],
      '#default_value' => $config->get('request_token_version'),
    ];

Where the options have keys "0" for "v1" and "1" for "v2"

But the module checks for "v1" and "v2" keys during the Token request:

private function requestToken($clientId, $clientSecret) {
    try {
      \Drupal::logger(__METHOD__)->info('%message', ['%message' => 'Fetching a new token.']);
      $url = $this->config
        ->get('request_token_url');
      $requestVersion = $this->config
        ->get('request_token_version');

      switch ($requestVersion) {
        case 'v1':
          $url .= '/v1/requestToken';
          $formParams = [
            'clientId' => $clientId,
            'clientSecret' => $clientSecret,
          ];
          $accessToken = 'accessToken';
          break;

        case 'v2':
        default:
          $url .= '/v2/token';
          $formParams = [
            'client_id' => $clientId,
            'client_secret' => $clientSecret,
            'grant_type' => 'client_credentials',
          ];
          $accessToken = 'access_token';
          break;
      }

So in the result default case ("v2") is always using.

Proposed resolution

Replace values "0" and "1" by values "v1" and "v2" on the settings form and update existing configs.