Update rubocop-govuk requirement from ~> 5.0.1 to ~> 5.1.6

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@v5.0.1...v5.1.6)

---
updated-dependencies:
- dependency-name: rubocop-govuk
  dependency-version: 5.1.6
  dependency-type: direct:development
...

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

Author: dependabot[bot]

govuk_design_system_formbuilder.gemspec

@@ -37,7 +37,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency("debug")
   s.add_development_dependency("rspec-html-matchers", "~> 0")
   s.add_development_dependency("rspec-rails", "~> 6.0")
-  s.add_development_dependency("rubocop-govuk", "~> 5.0.1")
+  s.add_development_dependency("rubocop-govuk", "~> 5.1.10")
   s.add_development_dependency("simplecov", "~> 0.20")
 
   # Required for the guide