Java: produce Java 8 class files for compatibility with packaged Java 11 binary

java/kotlin-extractor/build.gradle

@@ -29,4 +29,10 @@ task getHomeDir {
     doLast {
         println gradle.gradleHomeDir
     }
-}
+}
+
+java {
+  toolchain {
+    languageVersion.set(JavaLanguageVersion.of(8))
+  }
+}