Drop PECL YAML library support in favor of only Symfony's YAML?

Problem/Motivation

Follow-up from #3199691: [Symfony 6] Symfony no longer parses octal numbers starting with 0 alone.

PECL YAML uses libYAML, and libYAML is in the process of adding YAML 1.2 support. https://github.com/yaml/libyaml/issues/20

Steps to reproduce

Proposed resolution

Keep an eye on libYAML, and track any divergency/workarounds in the meantime that we might be able to undo once both YAML parses have 1.2 support.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet