chore(dev): various smaller fixes for tests and php-cs-fixer

[mfn]

Summary

• The underlying webonxy library added a new oneOf directive which made the test fail, although it isn't the focus of the test -> fixed
• php-cs-fixer: apply latest changes
• php-cs-fixer: enable parallel cores
• update phpstan baseline

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Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist:

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style