Similar to b744372, this defers loading
`ActionDispatch::Response` until after initialization, which will allow
applications to boot a bit faster in development but also paves the way
for `return_only_media_type_on_content_type` to work correctly when set
from `new_framework_defaults_6_0.rb`.

Benchmark:

    $ cat test.rb
    require "bundler/setup"
    before = ObjectSpace.each_object(Module).count
    start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
    require "action_controller"
    finish = Process.clock_gettime(Process::CLOCK_MONOTONIC)
    after = ObjectSpace.each_object(Module).count
    puts "took #{finish - start} and created #{after - before} modules"

Before:

    $ ruby test.rb
    took 0.35654300000169314 and created 608 modules

After:

    $ ruby test.rb
    took 0.2770050000108313 and created 466 modules

Co-authored-by: Serena Fritsch <serena@intercom.io>