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.

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