Update nokogiri requirement from ~> 1.8.4 to ~> 1.16.6

Updates the requirements on [nokogiri](https://github.com/sparklemotion/nokogiri) to permit the latest version.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/v1.16.6/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.8.4...v1.16.6)

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

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

rspec_api_documentation.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "webmock", "~> 3.23.0"
   s.add_development_dependency "rspec-its", "~> 1.3.0"
   s.add_development_dependency "faraday", "~> 1.0.0"
-  s.add_development_dependency "nokogiri", "~> 1.8.4"
+  s.add_development_dependency "nokogiri", "~> 1.16.6"
   s.add_development_dependency "yard", "~> 0.9.15"
   s.add_development_dependency "inch", "~> 0.8.0"
   s.add_development_dependency "minitest", "~> 5.8.4"