Remove require 'io/wait' where it's no longer necessary. (#6932)

* Remove `require 'io/wait'` as it's part of core now.

* Update ruby specs using version gates.

* Add note about why it's conditional.

ext/socket/lib/socket.rb

@@ -1,7 +1,11 @@
 # frozen_string_literal: true
 
 require 'socket.so'
-require 'io/wait' unless ::IO.method_defined?(:wait_readable, false)
+
+unless IO.method_defined?(:wait_writable, false)
+  # It's only required on older Rubies < v3.2:
+  require 'io/wait'
+end
 
 class Addrinfo
   # creates an Addrinfo object from the arguments.

spec/ruby/library/io-wait/wait_readable_spec.rb

@@ -1,6 +1,8 @@
 require_relative '../../spec_helper'
 
-require 'io/wait'
+ruby_version_is ''...'3.2' do
+  require 'io/wait'
+end
 
 describe "IO#wait_readable" do
   before :each do

spec/ruby/library/io-wait/wait_writable_spec.rb

@@ -1,6 +1,8 @@
 require_relative '../../spec_helper'
 
-require 'io/wait'
+ruby_version_is ''...'3.2' do
+  require 'io/wait'
+end
 
 describe "IO#wait_writable" do
   it "waits for the IO to become writable with no timeout" do

test/-ext-/thread_fd/test_thread_fd_close.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 require 'test/unit'
 require '-test-/thread_fd'
-require 'io/wait'
 
 class TestThreadFdClose < Test::Unit::TestCase
 

test/io/wait/test_io_wait.rb

@@ -3,10 +3,9 @@
 require 'test/unit'
 require 'timeout'
 require 'socket'
-begin
-  require 'io/wait'
-rescue LoadError
-end
+
+# For `IO#ready?` and `IO#nread`:
+require 'io/wait'
 
 class TestIOWait < Test::Unit::TestCase
 

test/io/wait/test_io_wait_uncommon.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: true
 require 'test/unit'
-require 'io/wait'
 
 # test uncommon device types to check portability problems
 # We may optimize IO#wait_*able for non-Linux kernels in the future

test/io/wait/test_ractor.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 require 'test/unit'
 require 'rbconfig'
-require 'io/wait'
 
 class TestIOWaitInRactor < Test::Unit::TestCase
   def test_ractor

test/ruby/test_process.rb

@@ -3,7 +3,6 @@
 require 'test/unit'
 require 'tempfile'
 require 'timeout'
-require 'io/wait'
 require 'rbconfig'
 
 class TestProcess < Test::Unit::TestCase

tool/lib/webrick/httpserver.rb

@@ -9,7 +9,6 @@
 #
 # $IPR: httpserver.rb,v 1.63 2002/10/01 17:16:32 gotoyuzo Exp $
 
-require 'io/wait'
 require_relative 'server'
 require_relative 'httputils'
 require_relative 'httpstatus'