SqlGatewayServiceITCase should utilize TestExecutorExtension instead of using a ThreadFactory

SqlGatewayServiceITCase uses a ExecutorThreadFactory for its asynchronous operations. Instead, one should use TestExecutorExtension to ensure proper cleanup of threads.

We might also want to remove the AbstractTestBase parent class because that uses JUnit4 whereas SqlGatewayServiceITCase is already based on JUnit5