Update rack-test requirement from ~> 0.6.3 to ~> 2.1.0

Updates the requirements on [rack-test](https://github.com/rack/rack-test) to permit the latest version.
- [Release notes](https://github.com/rack/rack-test/releases)
- [Changelog](https://github.com/rack/rack-test/blob/main/History.md)
- [Commits](rack/rack-test@v0.6.3...v2.1.0)

---
updated-dependencies:
- dependency-name: rack-test
  dependency-type: direct:development
...

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

rspec_api_documentation.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "aruba", "~> 0.14.14"
   s.add_development_dependency "capybara", "~> 3.39.2"
   s.add_development_dependency "rake", "~> 13.2.1"
-  s.add_development_dependency "rack-test", "~> 0.6.3"
+  s.add_development_dependency "rack-test", "~> 2.1.0"
   s.add_development_dependency "rack-oauth2", "~> 1.12.0"
   s.add_development_dependency "webmock", "~> 3.23.0"
   s.add_development_dependency "rspec-its", "~> 1.3.0"