KAPT: Run tests on JDK 21

Previously, some tests have also been disabled on JDK 11+, since
we did not filter out empty filesets, which led to failed assertion
inside JDK.
 #KT-57389

plugins/kapt4/build.gradle.kts

@@ -56,6 +56,7 @@ testsJar {}
 kaptTestTask("test", JavaLanguageVersion.of(8))
 kaptTestTask("testJdk11", JavaLanguageVersion.of(11))
 kaptTestTask("testJdk17", JavaLanguageVersion.of(17))
+kaptTestTask("testJdk21", JavaLanguageVersion.of(21))
 
 fun Project.kaptTestTask(name: String, javaLanguageVersion: JavaLanguageVersion) {
     val service = extensions.getByType<JavaToolchainService>()