Update rubocop-govuk requirement from = 4.14.0 to = 4.15.0

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@v4.14.0...v4.15.0)

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

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

plek.gemspec

@@ -19,5 +19,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency "climate_control"
   s.add_development_dependency "minitest"
   s.add_development_dependency "rake"
-  s.add_development_dependency "rubocop-govuk", "4.14.0"
+  s.add_development_dependency "rubocop-govuk", "4.15.0"
 end