Use -source/target 1.6 for Java sources in codegen tests

In cases when the test has the JVM_TARGET directive, use that one for
Java compilation as well.

Otherwise, for box tests, the corresponding test in `JvmTarget6OnJvm6`
(module `:compiler:tests-different-jdk`) will fail. However, it affects
all codegen tests, so fix a bunch of them which use Java 8+ features to
explicitly compile with JVM target 1.8. In particular, this obsoletes
the SKIP_JDK6 directive in those tests because "JVM_TARGET: 1.8" also
skips it for JDK 6.

The check for IS_SOURCE_6_STILL_SUPPORTED is needed in order to still be
able to run tests in the project while only having a single JDK > 11
installed, and having all of the env vars JDK_16, JDK_17, JDK_18
pointing to that JDK.

compiler/testData/codegen/box/delegation/byMiddleInterface.kt

@@ -1,6 +1,4 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
-
 // FILE: Base.java
 
 public interface Base {
@@ -12,6 +10,7 @@ public interface Base {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
 
 public interface BaseKotlin : Base {
 }

compiler/testData/codegen/box/delegation/defaultOverride.kt

@@ -1,6 +1,4 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
-
 // FILE: Base.java
 
 public interface Base {
@@ -12,6 +10,7 @@ public interface Base {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
 
 public interface BaseKotlin : Base {
     override fun getValue() = "OK"

compiler/testData/codegen/box/delegation/diamond.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Base.java
 
@@ -18,8 +17,8 @@ public interface Base2 extends Base {
     }
 }
 
-
 // FILE: main.kt
+// JVM_TARGET: 1.8
 
 interface KBase : Base
 

compiler/testData/codegen/box/delegation/diamond2.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Base.java
 
@@ -18,6 +17,7 @@ public interface Base2 extends Base {
 
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
 
 interface KBase : Base {
     override fun test() = "O" + getValue()

compiler/testData/codegen/box/delegation/inClassDeclaration.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Base.java
 
@@ -11,6 +10,8 @@ public interface Base {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
+
 class Fail : Base {
     override fun getValue() = "Fail"
 }

compiler/testData/codegen/box/delegation/mixed.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Base.java
 
@@ -11,6 +10,8 @@ public interface Base extends KBase {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
+
 interface KBase {
     fun getValue(): String
 

compiler/testData/codegen/box/delegation/simple.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Base.java
 
@@ -11,6 +10,8 @@ public interface Base {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
+
 class Fail : Base {
     override fun getValue() = "Fail"
 }

compiler/testData/codegen/box/delegation/simple1.0.kt

@@ -1,6 +1,5 @@
 // !LANGUAGE: -NoDelegationToJavaDefaultInterfaceMembers
 // IGNORE_BACKEND_FIR: JVM_IR
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Base.java
 
@@ -13,6 +12,8 @@ public interface Base {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
+
 class OK : Base {
     override fun getValue() = "OK"
 }

compiler/testData/codegen/box/javaInterop/notNullAssertions/enhancedNullability/inFunctionWithExpressionBodyWithJavaGeneric.kt

@@ -1,6 +1,5 @@
 // !LANGUAGE: +StrictJavaNullabilityAssertions
 // TARGET_BACKEND: JVM
-// SKIP_JDK6
 // See KT-8135
 // We could generate runtime assertion on call site for 'generic<NOT_NULL_TYPE>()' below.
 
@@ -24,11 +23,12 @@ fun <T> generic(j: J) = j.nullT<T>()
 import org.jetbrains.annotations.NotNull;
 
 public class J {
-    public <T> @NotNull T nullT() {
+    @NotNull
+    public <T> T nullT() {
         return null;
     }
 
     public void test() {
         TestKt.withAssertion(this);
     }
-}
+}

compiler/testData/codegen/box/jvm8/javaDefaults/defaultMethodOverride.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Simple.java
 
@@ -9,6 +8,7 @@ public interface Simple {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
 
 interface KInterface: Simple {
     override fun test(s: String): String {

compiler/testData/codegen/box/jvm8/javaDefaults/dontDelegateToDefaultMethods.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: Test.java
 
@@ -12,6 +11,7 @@ interface Test<T> {
 }
 
 // FILE: main.kt
+// JVM_TARGET: 1.8
 
 class Child : Test<String> {
     override fun call() : String {

compiler/testData/codegen/box/jvm8/javaDefaults/samOnInterfaceWithDefaultMethod.kt

@@ -1,4 +1,3 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
 // FILE: JavaCall.java
 
@@ -24,6 +23,7 @@ interface Test {
 }
 
 // FILE: sam.kt
+// JVM_TARGET: 1.8
 
 fun box(): String {
     val lambda = { "X" }

compiler/testData/codegen/box/smartCasts/kt19058.kt

@@ -1,5 +1,5 @@
-// SKIP_JDK6
 // TARGET_BACKEND: JVM
+// JVM_TARGET: 1.8
 // FILE: Test.kt
 open class KFoo {
     fun foo(): String {
@@ -17,4 +17,4 @@ public interface JBar {
     default String getBar() {
         return "OK";
     }
-}
+}

compiler/testData/codegen/box/traits/multipleImplFromJava.kt

@@ -1,5 +1,4 @@
 // TARGET_BACKEND: JVM
-// SKIP_JDK6
 // FILE: I.java
 interface I {
     default String ifun() { return "fail"; }
@@ -14,6 +13,7 @@ public class Z {
 public class Zz extends Z implements I {}
 
 // FILE: multipleImplFromJava.kt
+// JVM_TARGET: 1.8
 
 class Cc : Zz()