Drupal\BuildTests\Framework\DrupalMinkClient has been removed

To complete #3031379: Add a new test type to do real update testing, Drupal required Mink to follow refresh meta tags in tests. Symfony 4.2 introduced this functionality into BrowserKit so the Drupal code to handle meta refreshes is no longer required and this code has been removed in Drupal 9.1.0.

Any tests that required this functionality should have already been extending Drupal\BuildTests\Framework\BuildTestBase and are unaffected by this change. Any custom code that was instantiating Drupal\BuildTests\Framework\DrupalMinkClient directly should now use Behat\Mink\Driver\Goutte\Client instead.