The CI jobs for dev dependencies do not actually test dev dependencies

[stof]

The CI workflows configures the minimum-stability for this job: https://github.com/symfony/panther/blob/bc606efbca9b3189df90e52d10d655e2e52747d7/.github/workflows/ci.yml#L90

However, the committed composer.json already allows this stability while using prefer-stable: true: https://github.com/symfony/panther/blob/bc606efbca9b3189df90e52d10d655e2e52747d7/composer.json#L45-L46

a consequence of that setup is that https://github.com/symfony/panther/pull/682 did not actually ran the CI against Symfony 8.0.x-dev but against 7.3.6 in those jobs (and so missed the fact that the code is not actually compatible with Symfony 8, as reported in https://github.com/symfony/panther/issues/685)