Get to phpstan level 3

Problem/Motivation

Hi, thanks for the module! I'm maintaining another module that builds on this one (#2929260: Amazon Polly). When using phpstan to lint the code I came up against an issue where this module's docs weren't entirely accurate (eg. sometimes the doc mentions it might return false on failure, but doesn't indicate that in the actual @return).

Steps to reproduce

Run phpstan at level 3.

Proposed resolution

Fix phpstan level 3 issues and set the variable _PHPSTAN_LEVEL to 3 in .gitlab-ci.yml. We need to wait for #3609096: Error when using AWS profiles with an assumed role and STS AssumeRole fails to land to do that. Also ignore the errors that are fixed by that issue:

 ------ ------------------------------------------------------------------------------ 
  Line   src/Entity/Profile.php                                                        
 ------ ------------------------------------------------------------------------------ 
  347    Caught class Drupal\aws\Entity\Exception not found.                           
         🪪  class.notFound                                                            
  348    Call to method getMessage() on an unknown class Drupal\aws\Entity\Exception.  
         🪪  class.notFound                                                            
 ------ ------------------------------------------------------------------------------ 

Remaining tasks

There's a remaining issue: Method Drupal\aws\Entity\Form\ProfileDeleteConfirmForm::getDescription() should return Drupal\Core\StringTranslation\TranslatableMarkup but returns string. Currently it just returns the empty string (which works for now, but breaks the core interface contract and could feasibly stop working without warning in a future release). I noticed that the following code block used to exist in 23148a4cfe13fae2fddd49fe66bdaf65de795881:

/**
 * {@inheritdoc}
 * @todo
 */
public function getDescription() {
  $plugin_manager = \Drupal::service('plugin.manager.aws_service');
  $services = $plugin_manager->getDefinitions();
  foreach ($services as $service) {
    $config = $this->configFactory->get('aws.' . $service['id'] . '.settings');
    $profile_id = $config->get('profile');
    if ($profile_id === $this->entity->id()) {
      $affected_services[] = $profile_id;
    }
  }
  return $this->t('@services', ['@services' => $affected_services]);
  // Should return list of services that will be affected.
}

Should that be reinstated? Do we know why it was removed?

User interface changes

API changes

Data model changes