Fix PHPstan issues

Note commit 57b0b5ac adds arguments to the public contstructor of the GovUKNotifyMail plugin, which breaks BC.

The remaining phpstan warnings are duplicated across the three views filter plugins in this repeated function

  /**
   * {@inheritdoc}
   */
  public function addWhere($group, $field, $value = NULL, $operator = NULL) {
    // Ensure all variants of 0 are actually 0. Thus '', 0 and NULL are all
    // the default group.
    if (empty($group)) {
      $group = 0;
    }
    // Check for a group.
    if (!isset($this->where[$group])) {
      $this->setWhereGroup('AND', $group);
    }
    $this->where[$group]['conditions'][] = [
      'field' => $field,
      'value' => $value,
      'operator' => $operator,
    ];
  }

The phpstan warnings are

 ------ ---------------------------------------------------------------------------- 
  Line   modules/govuk_notify_views_backend/src/Plugin/views/filter/Id.php           
 ------ ---------------------------------------------------------------------------- 
  42     Call to an undefined method                                                 
         Drupal\govuk_notify_views_backend\Plugin\views\filter\Id::setWhereGroup().  
  44     Access to an undefined property                                             
         Drupal\govuk_notify_views_backend\Plugin\views\filter\Id::$where.           
         💡 Learn more:                                                              
            https://phpstan.org/blog/solving-phpstan-access-to-undefined-property    
 ------ ----------------------------------------------------------------------------