Update rack-test requirement from ~> 1.1 to ~> 2.0

Updates the requirements on [rack-test](https://github.com/rack/rack-test) to permit the latest version.
- [Release notes](https://github.com/rack/rack-test/releases)
- [Changelog](https://github.com/rack/rack-test/blob/main/History.md)
- [Commits](rack/rack-test@v1.1.0...v2.0.0)

---
updated-dependencies:
- dependency-name: rack-test
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

govuk_app_config.gemspec

@@ -30,7 +30,7 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "byebug"
   spec.add_development_dependency "climate_control"
-  spec.add_development_dependency "rack-test", "~> 1.1"
+  spec.add_development_dependency "rack-test", "~> 2.0"
   spec.add_development_dependency "rails", "~> 7"
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.10"