Remove the append property of the jacoco task extension

Gradle 6.0 removed JacocoTaskExtension.append: the JaCoCo agent now always appends, so setting it fails with Could not set unknown property 'append' for extension 'jacoco'. This recipe removes it from a jacoco { } block and drops a block left empty as a result. A build that set append = false keeps its execution data between runs afterwards, since Gradle offers no replacement. Kotlin scripts, where the property was isAppend on configure<JacocoTaskExtension> { }, are left unchanged. See the Gradle upgrade guide for more information.