Removed singleton dependency

  I have a plan to extract singleton library as bundled gems at Ruby 3.5.
  We should rewrite singleton logic with simple ruby code.

lib/webrick/utils.rb

@@ -87,7 +87,6 @@ def random_string(len)
     ###########
 
     require "timeout"
-    require "singleton"
 
     ##
     # Class used to manage timeout handlers across multiple threads.
@@ -116,8 +115,6 @@ def random_string(len)
     # will print 'foo'
     #
     class TimeoutHandler
-      include Singleton
-
       ##
       # Mutex used to synchronize access across threads
       TimeoutMutex = Thread::Mutex.new # :nodoc:
@@ -142,6 +139,11 @@ def self.terminate
         instance.terminate
       end
 
+      def self.instance
+        @instance ||= new
+      end
+      private_class_method :new
+
       ##
       # Creates a new TimeoutHandler.  You should use ::register and ::cancel
       # instead of creating the timeout handler directly.