make extension work with groovy 4.x and spock2.2-groovy-4.0

[mkleint]

Caused by: groovy.lang.MissingMethodException: No signature of method: org.dbunit.DataSourceDatabaseTester.getConnection() is applicable for argument types: () values: []
Possible solutions: getConnection()
	at be.janbols.spock.extension.dbunit.support.DbUnitInterceptor$1.getConnection(DbUnitInterceptor.groovy:72)
	at au.com.foxtel.registry.external.DataSourceForTests$_closure2.doCall(DataSourceForTests.groovy:84)
	at be.janbols.spock.extension.dbunit.support.DbUnitInterceptor.configureTester(DbUnitInterceptor.groovy:95)
	... 52 more
	```