Branch of drupal/core "8.6.x-dev" disappeared from packagist

Branch `8.6.x-dev` of `drupal/core` was removed from [packagist](https://packagist.org/packages/drupal/core). I opened a [ticket in DO](https://www.drupal.org/project/drupal/issues/3096237).

While this is not fixed, [branch 8.6.x](https://github.com/webflo/drupal-core-require-dev/blob/8.6.x/composer.json)'s requirements cannot be resolved.