Update cache format version to 7.1

[ryanb-gds]

Manuals Publisher has been running for some time now on 7.1, so it's probably safe to update the cache format.

Oddly when running RSpec I'm still seeing this deprecation warning:

DEPRECATION WARNING: Support for `config.active_support.cache_format_version = 6.1` has been deprecated and will be removed in Rails 7.2.

Check the Rails upgrade guide at https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#new-activesupport-cache-serialization-format
for more information on how to upgrade.
 (called from <top (required)> at /govuk/manuals-publisher/config/application.rb:19)

I think the reference to cache format version 6.1 might be included from a gem somehow, as manuals-publisher/config/application.rb:19 is

Bundler.require(*Rails.groups)