Fixup to usage of cgi/escape for Windows CI (#1378)

Fixed
https://github.com/ruby/rdoc/actions/runs/15687104051/job/44192968851?pr=1377

Author: hsbt

lib/rdoc/code_object/context/section.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require 'cgi/escape'
+require 'cgi/util' unless defined?(CGI::EscapeExt)
 
 ##
 # A section of documentation like:

lib/rdoc/code_object/method_attr.rb

@@ -283,6 +283,7 @@ def block_params=(value)
 
   def html_name
     require 'cgi/escape'
+    require 'cgi/util' unless defined?(CGI::EscapeExt)
 
     CGI.escape(@name.gsub('-', '-2D')).gsub('%', '-').sub(/^-/, '')
   end

lib/rdoc/markup/to_html.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 require 'cgi/escape'
-# For CGI.unescape on earlier rubies
-require 'cgi/util' if RUBY_VERSION < '3.5'
+require 'cgi/util' unless defined?(CGI::EscapeExt)
 
 ##
 # Outputs RDoc markup as HTML.

lib/rdoc/markup/to_label.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require 'cgi/escape'
+require 'cgi/util' unless defined?(CGI::EscapeExt)
 
 ##
 # Creates HTML-safe labels suitable for use in id attributes.  Tidylinks are