Add support for symfony/yaml version ^7

[ovidiuenache]

There have been some talks about removing this dependency from the list of required packages and there is an issue created regarding this (https://github.com/cebe/php-openapi/issues/170). However, there has been no update on it and I believe we should either remove it or keep it updated. This issue is focusing on the latter.

I'll open a PR about this.

[ovidiuenache]

https://github.com/cebe/php-openapi/pull/202

[ovidiuenache]

cc @cebe

[tacman]

Hi @cebe thanks for putting this library together. Any chance you could approve allowing the Symfony 7 YAML parser?

If you are going to release a new version, you could even just support php 8.1+ and Symfony 6/7.

[cebe]

#212 is merged with symfony 7 support.

[tacman]

thanks! Given how tiny the change is, can you tag it as a 1.7.1 release? (1.7.0 was in 2022)