Add -Xsuppress-deprecated-jvm-target-warning to modules compiled with 1.6

Currently this leads to an unknown argument warning, but it'll be
removed automatically on the next bootstrap.

core/descriptors.runtime/build.gradle.kts

@@ -29,8 +29,11 @@ val compileJava by tasks.getting(JavaCompile::class) {
 }
 
 val compileKotlin by tasks.getting(KotlinCompile::class) {
-    kotlinOptions.jvmTarget = "1.6"
-    kotlinOptions.jdkHome = rootProject.extra["JDK_16"] as String
+    kotlinOptions {
+        jvmTarget = "1.6"
+        jdkHome = rootProject.extra["JDK_16"] as String
+        freeCompilerArgs += "-Xsuppress-deprecated-jvm-target-warning"
+    }
 }
 
 val generateTests by generator("org.jetbrains.kotlin.generators.tests.GenerateRuntimeDescriptorTestsKt")
@@ -39,4 +42,4 @@ projectTest(parallel = true) {
     workingDir = rootDir
 }
 
-testsJar()
+testsJar()