Alter new upstream test for Fiber storage

* Remove the 3.2.3 lower bound, this spec fails on Ruby 3.2.0-3.2.2 (this file
  does not run on Ruby 3.1 and below)
* Run the test inside a new Fiber, we do not want to clutter the Fiber running
  the specs with additional keys.

core/fiber/storage_spec.rb

@@ -92,12 +92,11 @@
       end
     end
 
-    ruby_bug "#20978", "3.2.3"..."3.4" do
+    ruby_bug "#20978", ""..."3.4" do
       it "can use keys as strings" do
         key = Object.new
         def key.to_str; "Foo"; end
-        Fiber[key] = 42
-        Fiber["Foo"].should == 42
+        Fiber.new { Fiber[key] = 42; Fiber["Foo"] }.resume.should == 42
       end
     end