Allow using Symfony 7.0 dependencies

[javiereguiluz]

Fixes this problem:

 Problem 3
    - Root composer.json requires symfony-tools/docs-builder ^0.21 -> satisfiable by symfony-tools/docs-builder[v0.21.0].
    - symfony-tools/docs-builder v0.21.0 requires symfony/filesystem ^5.2 || ^6.0 -> found symfony/filesystem[v5.2.0-BETA1, ..., v5.4.25, v6.0.0-BETA1, ..., v6.4.0-BETA1] but these were not loaded, likely because it conflicts with another require.