KT-64385 Enable K2 KAPT by default

Also adds support for front-end compiler plugins in Kapt

plugins/kapt3/kapt3-cli/testData/integration/fallback/ap/META-INF/services/javax.annotation.processing.Processor

@@ -1 +0,0 @@
-apt.SampleApt

plugins/kapt3/kapt3-cli/testData/integration/fallback/ap/Processor.kt

@@ -1,34 +0,0 @@
-package apt
-
-import javax.annotation.processing.*
-import javax.lang.model.SourceVersion
-import javax.lang.model.element.TypeElement
-import javax.tools.Diagnostic.Kind.*
-import javax.tools.StandardLocation
-
-annotation class Anno
-
-class SampleApt : AbstractProcessor() {
-    override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean {
-        val writeKotlinFiles = processingEnv.options["kapt.test.writeKotlinFiles"] == "true"
-
-        for (element in roundEnv.getElementsAnnotatedWith(Anno::class.java)) {
-            val generatedSimpleName = element.simpleName.toString().capitalize()
-
-            val file = when (writeKotlinFiles) {
-                true -> processingEnv.filer.createResource(StandardLocation.SOURCE_OUTPUT, "generated", "$generatedSimpleName.kt")
-                false -> processingEnv.filer.createSourceFile("generated.$generatedSimpleName")
-            }
-
-            file.openWriter().use {
-                it.write("package generated;\npublic class $generatedSimpleName {}")
-            }
-        }
-
-        return true
-    }
-
-    override fun getSupportedOptions() = setOf("kapt.test.writeKotlinFiles")
-    override fun getSupportedSourceVersion() = SourceVersion.RELEASE_8
-    override fun getSupportedAnnotationTypes() = setOf("apt.Anno")
-}