CoreSession.setDocumentSystemProp must allow version write on VCS

Currently CoreSession.setDocumentSystemProp when trying to write on a version behaves differently on VCS and DBS:

• on VCS is prevents the write with a VersionNotModifiableException (actually only for the "isTrashed" prop for now)
• on DBS it allows the write

Given that this is for system properties, where the code is supposed to know what it does, we should always allow the write.