Mock Server integration

We are trying to integrate with Mock Server, we have the following code:

import static com.kms.katalon.core.checkpoint.CheckpointFactory.findCheckpoint
import static com.kms.katalon.core.testcase.TestCaseFactory.findTestCase
import static com.kms.katalon.core.testdata.TestDataFactory.findTestData
import static com.kms.katalon.core.testobject.ObjectRepository.findTestObject
import com.kms.katalon.core.checkpoint.Checkpoint as Checkpoint
import com.kms.katalon.core.cucumber.keyword.CucumberBuiltinKeywords as CucumberKW
import com.kms.katalon.core.mobile.keyword.MobileBuiltInKeywords as Mobile
import com.kms.katalon.core.model.FailureHandling as FailureHandling
import com.kms.katalon.core.testcase.TestCase as TestCase
import com.kms.katalon.core.testdata.TestData as TestData
import com.kms.katalon.core.testobject.TestObject as TestObject
import com.kms.katalon.core.webservice.keyword.WSBuiltInKeywords as WS
import com.kms.katalon.core.webui.keyword.WebUiBuiltInKeywords as WebUI
import internal.GlobalVariable as GlobalVariable

import org.mockserver.integration.ClientAndServer
import org.mockserver.model

ClientAndServer clientServer = ClientAndServer.startClientAndServer(2020);
System.out.println(clientServer.isRunning());

We have errors on run time at line
import org.mockserver.model

Katalon%20Studio/Test.prj/Scripts/REST%20examples/Simple%20examples/api-2-issue/Create%20issue/Create%20a%20new%20issue/Script1538639663709.groovy: 17: unable to resolve class org.mockserver.model
 @ line 17, column 1.
   import org.mockserver.model

and if we comment it at line
ClientAndServer clientServer = ClientAndServer.startClientAndServer(2020);

Create a new issue FAILED.
Reason:
java.lang.NoSuchFieldError: WRITE_BUFFER_WATER_MARK

We are using mockserver-netty-5.6.1-jar-with-dependencies.jar

Our bet is that the JRE that Katalon uses is 1.8, and we can’t upgrade it to JDK 12

Any help will be appreciated.

BR