Summary
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Add `-source` and `-target` and `--release` options to `javac` corresponding to JDK 21.

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 `21` as a valid argument to the options in question.

Specification
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Add support for  `javac -source 21 ...`, and `javac -target 21 ...`, and `javac --release 21 ...`.

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

As a consequence of adding support for the new argument to `javac`, support is also added to `javadoc` and the `java --source` option.