Complete AttributeDefinition.getImmutableFlags() -> getFlags() transition

Integration Jobs for wildfly-core are failing due to the getImmutableFlags() method removed by WFCORE-4611 requires adapting TransactionSubsystemRootResourceDefinition.java

To allow a smooth transition to the incoming wildfly-core release where this method is completely removed, we need to change full by using the deprecated one that uses an EnumSet as returned type in the method signature. This method signature used in full will be later compatible with the WFCORE-4611 code changes which make it returning a Set.