Remove SortedSet autoload and set/sorted_set

Implements [Feature #21287]

Author: jeremyevans

NEWS.md

@@ -105,6 +105,11 @@ The following bundled gems are updated.
 
     [[Feature #21258]]
 
+* With the move of `Set` from stdlib to core class, `set/sorted_set.rb` has
+  been removed, and `SortedSet` is no longer an autoloaded constant. Please
+  install the `sorted_set` gem and `require 'sorted_set'` to use `SortedSet`.
+  [[Feature #21287]]
+
 ## C API updates
 
 * IO
@@ -130,3 +135,4 @@ The following bundled gems are updated.
 [Bug #21049]:     https://bugs.ruby-lang.org/issues/21049
 [Feature #21216]: https://bugs.ruby-lang.org/issues/21216
 [Feature #21258]: https://bugs.ruby-lang.org/issues/21258
+[Feature #21287]: https://bugs.ruby-lang.org/issues/21287

lib/set/sorted_set.rb

@@ -26,8 +26,6 @@ def pp(*objs)
   private :pp
 end
 
-autoload :SortedSet, 'set/sorted_set'
-
 module Enumerable
   # Makes a set from the enumerable object with given arguments.
   def to_set(klass = Set, *args, &block)

prelude.rb

@@ -1,11 +1,13 @@
 require_relative '../../../spec_helper'
 
 describe "SortedSet" do
-  it "raises error including message that it has been extracted from the set stdlib" do
-    -> {
-      SortedSet
-    }.should raise_error(RuntimeError) { |e|
-      e.message.should.include?("The `SortedSet` class has been extracted from the `set` library")
-    }
+  ruby_version_is ""..."3.5" do
+    it "raises error including message that it has been extracted from the set stdlib" do
+      -> {
+        SortedSet
+      }.should raise_error(RuntimeError) { |e|
+        e.message.should.include?("The `SortedSet` class has been extracted from the `set` library")
+      }
+    end
   end
 end