use Yaml::decode() instead of Yaml::parse()

Problem/Motivation

phpstan is saying,

Avoid calling Symfony\Component\Yaml\Yaml::parse() directly. Use Drupal\Component\Serialization\Yaml::decode() instead, which handles exceptions consistently and applies the correct parse flags.

Steps to reproduce

Scan the module with phpstan or Upgrade Status under Drupal 11.

Proposed resolution

Use Yaml::decode() instead.

Remaining tasks

make an MR.