Currently, the process engine throws an exception if two activities with different behavior that call delegation code are mapped

For example, this is the case if a class delegate service task is mapped to an expression service task.

We should allow these cases by adding these behaviors as a class of equivalent behaviors to SameBehaviorInstructionValidator, see https://github.com/camunda/camunda-bpm-platform/blob/7.5.0-alpha4/engine/src/main/java/org/camunda/bpm/engine/impl/migration/validation/instruction/SameBehaviorInstructionValidator.java#L31-L42

Users can enable this behaviors already by configuring the validator themselves and replace it in the engine configuration.