Summary
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Add `-source` and `-target` and `--release` options to `javac` corresponding to JDK 16.

Problem
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Part of starting development on a new JDK is adding .new  `-source` and `-target` options to `javac`. These options are used in the compilation of the JDK sources.

Solution
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Add `16` as a valid argument to the options in question.

Specification
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Add support for  `javac -source 16 ...`, and `javac -target 16 ...`, and `javac --release 16 ...`.

The new value of -source/-target is also made the default.