Revert "Rack < v3 compatible release." (#20)

This reverts commit d4e8558 and restores the original implementation for Rack 3+.

Author: ioquatix

.github/workflows/test.yaml

@@ -48,4 +48,4 @@ jobs:
 
     - name: Run tests
       timeout-minutes: 10
-      run: bundle exec bake test
+      run: bundle exec rake test

lib/rack/session.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module Rack
+  module Session
+    autoload :Cookie, "rack/session/cookie"
+    autoload :Pool, "rack/session/pool"
+    autoload :Memcache, "rack/session/memcache"
+  end
+end