Declare DependencyLoader#new with ruby2_keywords to fix Ruby 3.2.0

When adding a middleware that receives keyword arguments in the
constructor, the call from `DependencyLoader#new` fails because
the method is not defined using `ruby2_keywords`.

This adds the required `ruby2_keywords` declaration to
`DependencyLoader#new`, fixing the tests and getting Faraday v1.x
working with Ruby 3.2.0.

Fixes lostisland#1479.

lib/faraday/dependency_loader.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'ruby2_keywords'
+
 module Faraday
   # DependencyLoader helps Faraday adapters and middleware load dependencies.
   module DependencyLoader
@@ -13,7 +15,7 @@ def dependency(lib = nil)
       self.load_error = e
     end
 
-    def new(*)
+    ruby2_keywords def new(*)
       unless loaded?
         raise "missing dependency for #{self}: #{load_error.message}"
       end