Flowable-Process-Validation: No validation error for parallel gateway flow with default attribute

We are using flowable 6.5.0.

When a parallel gateway flow of is marked as default flowable-process-validation successfully validates the bpmn process (see attached simplified example).

[ParallelGatewayDefaultAttribute.bpmn20.xml.txt](https://github.com/flowable/flowable-engine/files/4358445/ParallelGatewayDefaultAttribute.bpmn20.xml.txt)

The deployment then fails with:

```
o.f.e.i.b.d.ParsedDeploymentBuilder : Could not parse resource ParallelGatewayDefaultAttribute.bpmn
org.flowable.bpmn.exceptions.XMLException: cvc-complex-type.3.2.2: Attribute 'default' is not allowed to appear in element 'parallelGateway'.
at org.flowable.bpmn.converter.BpmnXMLConverter.convertToBpmnModel(BpmnXMLConverter.java:276)
at org.flowable.engine.impl.bpmn.parser.BpmnParse.execute(BpmnParse.java:148)
at org.flowable.engine.impl.bpmn.deployer.ParsedDeploymentBuilder.createBpmnParseFromResource(ParsedDeploymentBuilder.java:97)
at org.flowable.engine.impl.bpmn.deployer.ParsedDeploymentBuilder.build(ParsedDeploymentBuilder.java:55)
at org.flowable.engine.impl.bpmn.deployer.BpmnDeployer.deploy(BpmnDeployer.java:78)
at org.flowable.engine.impl.persistence.deploy.DeploymentManager.deploy(DeploymentManager.java:62)
...
Caused by: org.xml.sax.SAXParseException: cvc-complex-type.3.2.2: Attribute 'default' is not allowed to appear in element 'parallelGateway'.
at org.apache.xerces.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
at org.apache.xerces.util.ErrorHandlerWrapper.error(Unknown Source)
at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
at org.apache.xerces.impl.XMLErrorReporter.reportError(Unknown Source)
at org.apache.xerces.impl.xs.XMLSchemaValidator$XSIErrorReporter.reportError(Unknown Source)
at org.apache.xerces.impl.xs.XMLSchemaValidator.reportSchemaError(Unknown Source)
at org.apache.xerces.impl.xs.XMLSchemaValidator.processAttributes(Unknown Source)
at org.apache.xerces.impl.xs.XMLSchemaValidator.handleStartElement(Unknown Source)
at org.apache.xerces.impl.xs.XMLSchemaValidator.startElement(Unknown Source)
at org.apache.xerces.jaxp.validation.StAXValidatorHelper$StreamHelper.validate(Unknown Source)
at org.apache.xerces.jaxp.validation.StAXValidatorHelper.validate(Unknown Source)
at org.apache.xerces.jaxp.validation.ValidatorImpl.validate(Unknown Source)
at java.xml/javax.xml.validation.Validator.validate(Validator.java:124)
at org.flowable.bpmn.converter.BpmnXMLConverter.validateModel(BpmnXMLConverter.java:225)
at org.flowable.bpmn.converter.BpmnXMLConverter.convertToBpmnModel(BpmnXMLConverter.java:269)
... 144 common frames omitted
```