Use symfony/yaml ^5.0

[Zymna]

Hello,

Symfony has relased 5.0 of their yaml component:
https://github.com/symfony/yaml/blob/master/CHANGELOG.md
Changelog seems simple and tests run fine after requiring it.

This PR just updates composer files, I used composer 1.10.5:

openapi-reader$ composer require symfony/yaml ^5.0
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 1 update, 0 removals
  - Updating symfony/yaml (v4.2.8 => v5.0.7): Loading from cache
Writing lock file
Generating autoload files
1 package you are using is looking for funding.
Use the `composer fund` command to find out more!

Let me know if I something is wrong, thanks!