TASK: 5051 make subgraph not jsonSerializeable

[mhsdesign]

Resolves: https://github.com/neos/neos-development-collection/issues/5051

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Checklist

• [ ] Code follows the PSR-2 coding style
• [ ] Tests have been created, run and adjusted as needed
• [ ] The PR is created against the lowest maintained branch
• [ ] Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• [ ] Reviewer - The first section explains the change briefly for change-logs
• [ ] Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions