Release Note: Removal of javac Support for 6/1.6 source, target, and release Values

Consistent with the policy outlined in [JEP 182: Policy for Retiring javac -source and -target Options](https://openjdk.java.net/jeps/182), support for the 6/1.6 argument value for javac's `-source`, `-target`, and `--release` flags has been removed.