Opt in to the new public method if available

lib/bundler/rubygems_integration.rb

@@ -512,7 +512,7 @@ def replace_entrypoints(specs)
         h
       end
 
-      Gem::Specification.send(:default_stubs, "*.gemspec").each do |stub|
+      Bundler.rubygems.default_stubs.each do |stub|
         default_spec = stub.to_spec
         default_spec_name = default_spec.name
         next if specs_by_name.key?(default_spec_name)
@@ -857,6 +857,16 @@ def find_name(name)
         end
       end
 
+      if Gem::Specification.respond_to?(:default_stubs)
+        def default_stubs
+          Gem::Specification.default_stubs("*.gemspec")
+        end
+      else
+        def default_stubs
+          Gem::Specification.send(:default_stubs, "*.gemspec")
+        end
+      end
+
       def use_gemdeps(gemfile)
         ENV["BUNDLE_GEMFILE"] ||= File.expand_path(gemfile)
         require "bundler/gemdeps"