Update rack requirement from ~> 1.2 to ~> 2.0

Updates the requirements on [rack](https://github.com/rack/rack) to permit the latest version.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.2...2.0.6)

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

rack-strip-cookies.gemspec

@@ -19,6 +19,6 @@ Gem::Specification.new do |spec|
  spec.require_paths = ['lib']
 
  spec.add_development_dependency 'bundler', '~> 2.0'
- spec.add_development_dependency 'rack', '~> 1.2'
+ spec.add_development_dependency 'rack', '~> 2.0'
  spec.add_development_dependency 'rack-test', '~> 0.6.2'
 end