Use Rackup::Handler when available

The `Rack::Handler` was extracted from Rack 3+ into separate `rackup`
package.

Author: voxik

rb/spec/integration/selenium/webdriver/spec_support/rack_server.rb

@@ -18,6 +18,10 @@
 # under the License.
 
 require 'rack'
+begin
+  require 'rackup'
+rescue LoadError
+end
 
 module Selenium
   module WebDriver
@@ -76,7 +80,7 @@ def handler
 
           handler = handlers.find { |h| load_handler h }
           constant = handler == 'webrick' ? 'WEBrick' : handler.capitalize
-          Rack::Handler.const_get constant
+          defined?(Rackup) ? Rackup::Handler.const_get(constant) : Rack::Handler.const_get(constant)
         end
 
         def load_handler(handler)