Update Jekyll server monkey-patch to mirror Core

[ashmaroli]

Summary

• Mark the methods as private since #start_up_webrick is a private method in the upstream command class
• From Jekyll 3.7 onward, local variable server is an instance_variable in the overridden method upstream

Reference

The upstream method definition as of v4.0.0:

    def start_up_webrick(opts, destination)
      @reload_reactor.start(opts) if opts["livereload"]

      @server = WEBrick::HTTPServer.new(webrick_opts(opts)).tap { |o| o.unmount("") }
      @server.mount(opts["baseurl"].to_s, Servlet, destination, file_handler_opts)

      Jekyll.logger.info "Server address:", server_address(@server, opts)
      launch_browser @server, opts if opts["open_url"]
      boot_or_detach @server, opts
    end

[zocoi]

Just want to say thank you for the work to make this gem compatible with parent 4.0