Remove use of deprecated 'set-output' command

- Use GITHUB_OUTPUT file in init script
- Write to a marker file in configuratiion-cache tests

Fixes #461

.github/workflow-samples/groovy-dsl/build.gradle

@@ -11,8 +11,8 @@ dependencies {
 }
 
 tasks.named("test").configure {
-    // Echo an output value so we can detect configuration-cache usage
-    println "::set-output name=task_configured::yes"
+    // Write marker file so we can detect if task was configured
+    file("task-configured.txt").text = "true"
 
     doLast {
         if (System.properties.verifyCachedBuild) {