fix: Resolve the PHPStan findings raised by the updated analyzers

Problem/Motivation

The phpstan CI job fails on the 4.0.x, 3.1.x and 3.0.x branches: the updated phpstan-drupal analyzers raise drupal.symfonyYamlParse on pre-existing code in src/EventSubscriber/ActiveThemeChangeSubscriber.php.

Steps to reproduce

Run the phpstan job on the branch pipeline.

Proposed resolution

• Use \Drupal\Component\Serialization\Yaml::decode() / Yaml::encode() instead of the Symfony YAML parser and dumper, and catch InvalidDataTypeException.

Remaining tasks

• ✅ File an issue
• ❌ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit testing coverage
• ➖ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ❌ Readability
• ➖ Accessibility
• ➖ Performance
• ➖ Security
• ➖ Developer Documentation
• ➖ User Guide Documentation
• ➖ Reviewed by human
• ➖ Code review by maintainers
• ➖ Full testing and approval
• ➖ Credit contributors
• ➖ Review with the product owner
• ➖ Release notes snippet
• ✅ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Modernized the YAML handling in the active-theme change subscriber to the Drupal serialization component.

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