Fix NoMethodError: undefined method `hour' for 1:Integer

We have to `require "active_support/time"` to define the helper methods
we use to set the Cache-Control header in the [test environment][usage].

I don't know why bumping the govuk_publishing_components gem from
v23.10.1 to v23.12.1 has caused this issue in both [Static][]
and [Feedback][] (nor why the tests passed on the PR and only
subsequently failed after merge).

I write up an investigation in alphagov/static#2384,
but have nothing conclusive. It won't be a good use of time to
investigate too much further, so I'm happy that this is a harmless
change which unblocks us from deploying an important fix.

Trello: https://trello.com/c/YhIIykse/2291-3-fix-cross-site-scripting-vulnerabilities

[Static]: alphagov/static#2379
[Feedback]: #1149
[usage]: https://github.com/alphagov/feedback/blob/1cfe51bc39ec48f2cba0c502b15720fdc4dbbcd2/config/environments/test.rb#L14

config/application.rb

@@ -3,6 +3,7 @@
 require "rails"
 require "action_controller/railtie"
 require "action_mailer/railtie"
+require "active_support/time"
 require "sprockets/railtie"
 
 Bundler.require(*Rails.groups)