Update ezt tool so GWT build can run under Java 17

[niloc132]

As of #9780 being merged, GWT can be built with Java 8-11, and runs successfully on Java 8-latest (though legacy dev mode may not work with newer versions of Java, or may require additional JVM arguments to circumvent JPMS checks).

It looks like rewriting the Ezt doc tool is the last remaining piece to support Java 17. That is, providing a new mechanism to generate the contents of https://www.gwtproject.org/doc/latest/RefJreEmulation.html, the HTML contents after the header of https://github.com/gwtproject/gwt-site/blob/main/src/main/markdown/doc/latest/RefJreEmulation.md?plain=1.

The classes responsible for this are com.google.doctool.JreDocTool, the main() invoked by the target emul-ezt in doc/build.xml, a builder class com.google.doctool.JreDocToolFactory, and the class com.google.doctool.custom.EztDoclet - apparently used as a doclet to emit a plain HTML file when run.

In a replacement, ideally the HTML structure would follow the same general format (to make it easier to confirm that the contents are the same, through a git diff), and hopefully would preserve any html anchors as well (to avoid breaking inbound links). I don't see a reason that the new version should necessarily be implemented as a doclet though, as long as the functionality is otherwise consistent.

To run this today, invoke

ant doc

The output can then be found at build/out/doc/emul-ezt/fragment.html.