Explicitly require rack/server as it may not be part of rack.

Rack 3 separates this code into a separate gem, so autoloads from `rack`
may not load it. Explicitly require it where needed.

railties/lib/rails/commands/server/server_command.rb

@@ -6,6 +6,7 @@
 require "active_support/core_ext/string/filters"
 require "rails/dev_caching"
 require "rails/command/environment_argument"
+require "rack/server"
 
 module Rails
   class Server < ::Rack::Server