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/bytecodeListing/kt43217.kt

@@ -1,4 +1,5 @@
 // FILE: kt43217.kt
+// JVM_TARGET: 1.8
 class A {
     private val b =
         object : DoubleExpression() {
@@ -43,4 +44,3 @@ public interface ObservableDouble extends ObservableValue<Double> {
     @NotNull
     Object isEqualTo(double value);
 }
-