Add ostruct as an explicit dependency

Ruby is dropping ostruct from standard library and will require it to be
installed as a gem. When using an app with Ruby 3.3.5 there is a
warning:

```
/root/.rbenv/versions/3.3.5/lib/ruby/3.3.0/bundled_gems.rb:75: warning: /root/.rbenv/versions/3.3.5/lib/ruby/3.3.0/ostruct.rb was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
Also please contact the author of govuk_publishing_components-43.3.0 to request adding ostruct into its gemspec.
```

CHANGELOG.md

@@ -7,6 +7,10 @@
   useful summary for people upgrading their application, not a replication
   of the commit log.
 
+## Unreleased
+
+* Add `ostruct` as an explicit dependency ([PR #4251](https://github.com/alphagov/govuk_publishing_components/pull/4251))
+
 ## 43.4.1
 
 * Add accessible-autocomplete node_modules to gemspec ([PR #4246](https://github.com/alphagov/govuk_publishing_components/pull/4246))

Gemfile.lock

@@ -5,6 +5,7 @@ PATH
       govuk_app_config
       govuk_personalisation (>= 0.7.0)
       kramdown
+      ostruct
       plek
       rails (>= 6)
       rouge
@@ -423,6 +424,7 @@ GEM
       opentelemetry-semantic_conventions
     opentelemetry-semantic_conventions (1.10.1)
       opentelemetry-api (~> 1.0)
+    ostruct (0.6.0)
     parallel (1.25.1)
     parser (3.3.3.0)
       ast (~> 2.4.1)

govuk_publishing_components.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |s|
   s.add_dependency "govuk_app_config"
   s.add_dependency "govuk_personalisation", ">= 0.7.0"
   s.add_dependency "kramdown"
+  s.add_dependency "ostruct"
   s.add_dependency "plek"
   s.add_dependency "rails", ">= 6"
   s.add_dependency "rouge"