Remove javadoc support for HTML 4.

Summary

Remove the -html4 option.

Problem

The world is moving to HTML 5; javadoc has supported HTML 5 since JDK 9, and warned about the use of HTML 4 since JDK 11. It is a maintenance overhead to support both versions, and there is little reason to do so.

Solution

Remove support for HTML 4, and with it the -html4 option.

The html5 option will remain, but will become a no-op, since HTML 5 is the only output format used. The command-line help and man page should be updated to note the option is just retained for compatibility and is no longer necessary to be used.

javac supports a related option:

  --doclint-format {html4,html5}
        Specify the format for documentation comments

We should remove html4 from the set of applicable values, and update the command-line help and man page.

Specification

The html4 option for the standard doclet will be removed.

The html4 value for the javac --doclet-format option will be removed.

Attached diff of Javac and Javadoc man pages.