Update the data fixtures

[rosier]

The class parameter is now mandatory for references.

Fixes exception when running bin/console doctrine:fixtures:load:

[ArgumentCountError]
  Too few arguments to function Doctrine\Common\DataFixtures\AbstractFixture::getReference(), 1 passed in /workspace/project/src/DataFi
  xtures/AppFixtures.php on line 143 and exactly 2 expected

Related to: doctrine/data-fixtures#464

[javiereguiluz]

Nice changes! Thanks a lot Jan for fixing this.