in favour of Constants.ClassFileVersion.of(int, int). The few ones
retained are used from production code, the vast majority of removed
ones only from test code.

This gets rid of yet another 'AspectJ_JDK_Update' tag. One less place to
check and update with each newly supported Java version. :-)

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

This was required by the Eclipse team as one precondition for the next
release.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

- Test all features which were preview in 14+15 and are now final in 16,
  compiling them with language level 16.
- For Java 15 we only have sanity tests (and of course the Java <14
  tests), compiling Java 16 features to target 15 does not seem to work.
- Test remaining Java 16 preview feature (sealed classes).
- Instead of overriding runTest(String) in several base classes like
  XMLBasedAjcTestCaseForJava*Only or XMLBasedAjcTestCaseForJava*OrLater,
  we now override setUp() from JUnit's TestCase base class. This will
  run before runTest(String) and make the tests fail much faster, if a
  user tries to run them on the wrong VM.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

- Java 14 feature sample classes moved from 'bugs' to 'features'
- One test case using a Java 14 preview feature was moved to the
  Java 14-only tests

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

Some Ajc196 tests are using preview features (see .../ajc196.xml), i.e.
they will fail on Java 15+ because code compiled with '--enable-preview'
can only run on the same JVM version, not on a more recent one. Hence,
the preview-using tests are now being excluded in order to make the
build run on Java 15, even though no Java 15 features are present in the
current 1.9.7 snapshot.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>