Update rubocop-govuk requirement from ~> 4.2.0 to ~> 4.3.0

Updates the requirements on [rubocop-govuk](https://github.com/alphagov/rubocop-govuk) to permit the latest version.
- [Release notes](https://github.com/alphagov/rubocop-govuk/releases)
- [Changelog](https://github.com/alphagov/rubocop-govuk/blob/main/CHANGELOG.md)
- [Commits](alphagov/rubocop-govuk@v4.2.0...v4.3.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

govuk_design_system_formbuilder.gemspec

@@ -37,7 +37,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency("pry-byebug", "~> 3.9", ">= 3.9.0")
   s.add_development_dependency("rspec-html-matchers", "~> 0")
   s.add_development_dependency("rspec-rails", "~> 5.0")
-  s.add_development_dependency("rubocop-govuk", "~> 4.2.0")
+  s.add_development_dependency("rubocop-govuk", "~> 4.3.0")
   s.add_development_dependency("simplecov", "~> 0.20")
 
   # Required for the guide