Use Rack::Files for Rack 3+ compatibility

Rack has renamed `Rack::File` to `Rack::Files` starting with Rack 2.1.0
[[1]]. Rack 3+ drops the old `Rack::File` class and therefore there are
bunch of errors such as:

~~~
  334) Selenium::WebDriver::VirtualAuthenticator#user_verified= can not obtain credential requiring verification when set to false
       Failure/Error: @static = Rack::File.new(file_root)

       NameError:
         uninitialized constant Rack::File
       # ./spec/integration/selenium/webdriver/spec_support/rack_server.rb:103:in 'Selenium::WebDriver::SpecSupport::RackServer::TestApp#initialize'
       # ./spec/integration/selenium/webdriver/spec_support/rack_server.rb:30:in 'Class#new'
       # ./spec/integration/selenium/webdriver/spec_support/rack_server.rb:30:in 'Selenium::WebDriver::SpecSupport::RackServer#initialize'
       # ./spec/integration/selenium/webdriver/spec_support/test_environment.rb:87:in 'Class#new'
       # ./spec/integration/selenium/webdriver/spec_support/test_environment.rb:87:in 'Selenium::WebDriver::SpecSupport::TestEnvironment#app_server'
       # ./spec/integration/selenium/webdriver/spec_support/test_environment.rb:162:in 'Selenium::WebDriver::SpecSupport::TestEnvironment#url_for'
       # ./spec/integration/selenium/webdriver/spec_support/helpers.rb:41:in 'Selenium::WebDriver::SpecSupport::Helpers#url_for'
       # ./spec/integration/selenium/webdriver/virtual_authenticator_spec.rb:55:in 'block (2 levels) in <module:WebDriver>'
~~~

[1]: rack/rack@626272b

Author: voxik

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

@@ -100,7 +100,7 @@ class TestApp
           BASIC_AUTH_CREDENTIALS = %w[test test].freeze
 
           def initialize(file_root)
-            @static = Rack::File.new(file_root)
+            @static = Rack::Files.new(file_root)
           end
 
           def call(env)