Update rubocop-govuk requirement from = 5.0.3 to = 5.0.4

Updates the requirements on [rubocop-govuk](https://github.com/alphagov/rubocop-govuk) to permit the latest version.
- [Changelog](https://github.com/alphagov/rubocop-govuk/blob/main/CHANGELOG.md)
- [Commits](alphagov/rubocop-govuk@v5.0.3...v5.0.4)

---
updated-dependencies:
- dependency-name: rubocop-govuk
  dependency-type: direct:development
...

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

govuk_app_config.gemspec

@@ -39,7 +39,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.10"
   spec.add_development_dependency "rspec-its", "~> 2.0"
-  spec.add_development_dependency "rubocop-govuk", "5.0.3"
+  spec.add_development_dependency "rubocop-govuk", "5.0.4"
   spec.add_development_dependency "simplecov"
   spec.add_development_dependency "webmock"
 end