Symfony YAML does not parse all YAML

While trying to add more tests for more specs from these sources:

- https://github.com/Mermade/openapi3-examples/tree/master/resources
- https://github.com/Nexmo/api-specification/tree/master/definitions
- https://github.com/APIs-guru/openapi-directory

I found that symfony YAML fails on a lot of these.
We either need an alternative YAML parser or improve symfony YAML to not fail.

- https://github.com/yaml/yaml-test-suite
- https://github.com/symfony/symfony/issues/27965

Reproduce the failures using the OpenAPI examples and definitions linked in the issue, then compare the behavior with the yaml-test-suite and Symfony issue #27965. Determine whether the Symfony YAML parser should be improved or replaced, and verify that the affected OpenAPI YAML files parse successfully before considering the issue done.

Written by the indexing model from the issue text.