Don't use Rack::Lock for concurrent but not multithread configuration…

… on Ruby 2

Ruby 2 Mutex only works for multiple threads, not multiple fibers.

Such apps are probably still broken at runtime unless they are
using their own locks, but this matches the Rack 2 behavior.

lib/rack/lock.rb

@@ -14,7 +14,7 @@ def call(env)
     end
 
     def self.rackup(config, app)
-      if config.multithread?
+      if config.multithread? || (config.concurrent? && RUBY_VERSION >= '3')
         new(app)
       else
         app