Update rubocop-govuk requirement from = 4.12.0 to = 4.13.0

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@v4.12.0...v4.13.0)

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

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

govspeak.gemspec

@@ -42,6 +42,6 @@ library for use in the UK Government Single Domain project'
   s.add_development_dependency "minitest", "~> 5.14"
   s.add_development_dependency "pry-byebug"
   s.add_development_dependency "rake"
-  s.add_development_dependency "rubocop-govuk", "4.12.0"
+  s.add_development_dependency "rubocop-govuk", "4.13.0"
   s.add_development_dependency "simplecov"
 end