Update rubocop-rails requirement from ~> 2.20.2 to ~> 2.21.1

Updates the requirements on [rubocop-rails](https://github.com/rubocop/rubocop-rails) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-rails@v2.20.2...v2.21.1)

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

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

publify_core.gemspec

@@ -60,7 +60,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rubocop-capybara", "~> 2.19.0"
   s.add_development_dependency "rubocop-factory_bot", "~> 2.24.0"
   s.add_development_dependency "rubocop-performance", "~> 1.19.0"
-  s.add_development_dependency "rubocop-rails", "~> 2.20.2"
+  s.add_development_dependency "rubocop-rails", "~> 2.21.1"
   s.add_development_dependency "rubocop-rspec", "~> 2.24.1"
   s.add_development_dependency "shoulda-matchers", "~> 5.3"
   s.add_development_dependency "simplecov", "~> 0.22.0"