Always use YamlSymfony for loading service yaml files

Problem/Motivation

From #3108309-29: Support Yaml::PARSE_CUSTOM_TAGS in \Drupal\Component\Serialization\YamlSymfony::decode

Re #27 I was tempted to suggest that. I was reticent because service.yml files are one of the thing parsed on cold cache. But the more I think about it the more I think that this is a good approach for now. The capabilities of the YAML parser and container configuration are quite closely related so this does not feel like a bad decision - also we use FileCache here so potentially this has no impact on cold cache as that can persist across a cache rebuild.

Trying to duplicate custom tag support from upstream Symfony in both the YamlSymfony and YamlPecl serializers is becoming increasingly complex. Let's save ourselves the headache and only use YamlSymfony for service yamls.

Steps to reproduce

Proposed resolution

Only use YamlSymfony for service yamls.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet