Fix testing against 6.x

Author: jjohannes

build.gradle.kts

@@ -61,6 +61,8 @@ pluginPublishConventions {
 
 testingConventions { testGradleVersions("6.8.3", "6.9.4", "7.0.2", "8.0.2", "8.14.3") }
 
+tasks.compileTestJava { options.release = 11 } // allow tests to run against 6.x
+
 val generateCapabilitiesList =
     tasks.register<CapabilityListing>("generateCapabilitiesList") {
         pluginClasses.from(tasks.jar)
@@ -114,5 +116,7 @@ abstract class CapabilityListing : DefaultTask() {
 // === the following custom configuration should be removed once tests are migrated to Java
 apply(plugin = "groovy")
 
+tasks.named<GroovyCompile>("compileTestGroovy") { targetCompatibility = "11" } // allow tests to run against 6.x
+
 dependencies { testImplementation("org.spockframework:spock-core:2.3-groovy-4.0") } //
 // ====================================================================================