Update CollectScreenshotsTask to handle Kotlin 2.1.20 changes

Kotlin 2.1.20 altered the structure of output.classesDirs, which can no longer assume a single file. Updated the task to filter for the correct path, ensuring compatibility with the new Kotlin version.

buildSrc/src/main/kotlin/org/openrndr/extra/convention/kotlin-multiplatform.gradle.kts

@@ -55,7 +55,8 @@ kotlin {
             val demo by creating {
                 associateWith(main)
                 tasks.register<CollectScreenshotsTask>("collectScreenshots") {
-                    inputDir.set(output.classesDirs.singleFile)
+                    // since Kotlin 2.1.20 output.classesDirs no longer contains a single file
+                    inputDir.set(output.classesDirs.filter { it.path.contains("classes/kotlin") }.singleFile)
                     runtimeDependencies.set(runtimeDependencyFiles)
                     outputDir.set(project.file(project.projectDir.toString() + "/images"))
                     dependsOn(compileTaskProvider)