Update gherkin requirement from ~> 3.2.0 to ~> 9.0.0

Updates the requirements on [gherkin](https://github.com/cucumber/cucumber) to permit the latest version.
- [Release notes](https://github.com/cucumber/cucumber/releases)
- [Commits](cucumber/common@react/v3.2.0...gherkin/v9.0.0)

---
updated-dependencies:
- dependency-name: gherkin
  dependency-type: direct:development
...

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

rspec_api_documentation.gemspec

@@ -34,7 +34,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "inch", "~> 0.8.0"
   s.add_development_dependency "minitest", "~> 5.8.4"
   s.add_development_dependency "contracts", "~> 0.17"
-  s.add_development_dependency "gherkin", "~> 3.2.0"
+  s.add_development_dependency "gherkin", "~> 9.0.0"
   s.add_development_dependency "multi_json", "~> 1.11.2"
   s.add_development_dependency "rspec", "~> 3.0"