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.
Dmitriy Novozhilov