Remove javac support for 6/1.6 source and target values

Summary

Remove support for the 6/1.6 argument to javac's -source, -target, and --release options.

Problem

Per the policy described in

JEP 182: Policy for Retiring javac -source and -target Options http://openjdk.java.net/jeps/182

support for a particular -source/-target/--release value has a limited lifetime. After discussion on jdk-dev (http://mail.openjdk.java.net/pipermail/jdk-dev/2018-May/001190.html), removing support for 6 was deferred from JDK 11 to JDK 12.

Solution

Drop support for 6 and have 7 be reported as obsolete.

Specification

No longer accept 6/1.6 as a valid argument and when used report 7/1.7 as obsolete.