Support Rack 3.x

[lloeki]

From #389 here's a first draft that makes Thin specs pass.

There's a warning that should probably be addressed:

warning: Rack::File is deprecated and will be removed in Rack 3.1

rg Rack::File
lib/rack/adapter/rails.rb
26:        @file_app  = Rack::File.new(::File.join(RAILS_ROOT, "public"))

lib/rack/adapter/loader.rb
67:        return Rack::File.new(options[:chdir])

example/adapter.rb
23:    run Rack::File.new('.')
30:#                          '/files' => Rack::File.new('.'))

spec/rack/loader_spec.rb
35:    expect(Rack::File).to receive(:new)