Stop adding drupal/core-dev and phpspec/prophecy-phpunit, tests cannot be executed in sandboxes

Problem / Motivation

We do not need to add these extra dependencies, since no development operations are allowed (running tests, PHPStan, etc.)

Code to remove

```
      // We need to require drupal/core and drupal/core-dev at the requested
      // Drupal core version, otherwise `composer update` could bump the
      // versions to the latest available one.
      $commands[] = sprintf('cd stm && composer require --dev --no-update drupal/core:%1$s drupal/core-dev:%1$s', $parameters['drupal_core_version']);
      // The phpspec/prophecy-phpunit check was added in 9.1.6
      // @see https://www.drupal.org/i/3182653
      // @see https://git.drupalcode.org/project/drupal/-/commit/94d0c1f
      if (Semver::satisfies($parameters['drupal_core_version'], '>=9.1.6')) {
        $commands[] = 'cd stm && composer require --dev --no-update phpspec/prophecy-phpunit:^2';
      }
```