Update rack-attack requirement from ~> 6.7.0 to ~> 6.8.0 (#223)

Updates the requirements on [rack-attack](https://github.com/rack/rack-attack) to permit the latest version.
- [Release notes](https://github.com/rack/rack-attack/releases)
- [Changelog](https://github.com/rack/rack-attack/blob/main/CHANGELOG.md)
- [Commits](rack/rack-attack@v6.7.0...v6.8.0)

---
updated-dependencies:
- dependency-name: rack-attack
  dependency-version: 6.8.0
  dependency-type: direct:development
...

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Author: dependabot[bot]

cloudflare-rails.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'appraisal', '~> 2.5.0'
   spec.add_development_dependency 'bundler', '>= 2.4.18'
   spec.add_development_dependency 'pry-byebug'
-  spec.add_development_dependency 'rack-attack', '~> 6.7.0'
+  spec.add_development_dependency 'rack-attack', '~> 6.8.0'
   spec.add_development_dependency 'rake', '~> 13.2.1'
   spec.add_development_dependency 'rspec', '~> 3.13.0'
   spec.add_development_dependency 'rspec-rails', '~> 7.1.1'