Add support for symfony/yaml:6.0

[victorstanciu]

First of all, thank you for this great library ♥

Symfony 6.0 has been released recently, but any projects that depend on cebe/php-openapi can't be upgraded because the dependency on symfony/yaml is limited to ^3.4 | ^4.0 | ^5.0.

[victorstanciu]

Ah, never mind, I just noticed there's already an active PR for this.