Update description and parameters of --compress to reflect JDK21 changes

$ jlink --help

[…]

      --compress <compress>             Compression to use in compressing resources:
                                        Accepted values are:
                                        zip-[0-9], where zip-0 provides no compression,
                                        and zip-9 provides the best compression.
                                        Default is zip-6.
                                        Deprecated values to be removed in a future release:
                                        0:  No compression. Equivalent to zip-0.
                                        1:  Constant String Sharing
                                        2:  Equivalent to zip-6.

    /**
     * Here you can define the compression of the resources being used. The command line equivalent is:
     * <code>-c, --compress=level&gt;</code>. The valid values for the level are: <code>0, 1, 2</code>.
     */
    @Parameter
    private Integer compress;