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/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);
     }
-}
+}