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

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

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