Use Struct instead of OpenStruct.

I will migrate osturct as bundled gems at Ruby 3.5. We should remove
its dependency from Rake

lib/rake.rb

@@ -30,7 +30,6 @@ module Rake; end
 require "singleton"
 require "monitor"
 require "optparse"
-require "ostruct"
 
 require "rake/ext/string"
 

lib/rake/application.rb

@@ -165,7 +165,15 @@ def add_loader(ext, loader)
 
     # Application options from the command line
     def options
-      @options ||= OpenStruct.new
+      return @options if @options
+
+      @options = Struct.new(
+        :always_multitask, :backtrace, :build_all, :dryrun,
+        :ignore_deprecate, :ignore_system, :job_stats, :load_system,
+        :nosearch, :rakelib, :show_all_tasks, :show_prereqs,
+        :show_task_pattern, :show_tasks, :silent, :suppress_backtrace_pattern,
+        :thread_pool_size, :trace, :trace_output, :trace_rules
+      ).new
     end
 
     # Return the thread pool used for multithreaded processing.