Make \Drupal\form_test\Plugin\Block\RedirectFormBlock cachable

Problem/Motivation

In #3123832: [META] Fix @todo items referencing closed issues we've discovered there's a @todo in \Drupal\form_test\Plugin\Block\RedirectFormBlock::getCacheMaxAge.
This @todo is linked to an d.o. issue that is already closed: #2351015: URL generation does not bubble cache contexts.

This is the @todo:

  /**
   * {@inheritdoc}
   *
   * @todo Make cacheable once https://www.drupal.org/node/2351015 lands.
   */
  public function getCacheMaxAge() {
    return 0;
  }

Steps to reproduce

Proposed resolution

Remove the getCacheMaxAge() in this class.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet