BUG: 9.0 Allow default value for reference properties again

[mhsdesign]

https://neos-project.slack.com/archives/C3MCBK6S2/p1688542588698759

currently this was "disabled" due to our more strict separation of properties vs references.

But as its also not possible to set subtree tags or the hidden / disabled state one might argue this is too complex for a default value and not without extension.
In #4484 i argued that node templates has to be used for this case.

[mhsdesign]

imo, we hardly need this in the core but this can be archived via node templates as references are of complex nature and not a simple "defaultValue" https://github.com/Flowpack/Flowpack.NodeTemplates

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the real question is how can we enforce and if we should that defaultValue is not set for references