Update rack-oauth2 requirement from ~> 1.12.0 to ~> 1.21.3

Updates the requirements on [rack-oauth2](https://github.com/nov/rack-oauth2) to permit the latest version.
- [Release notes](https://github.com/nov/rack-oauth2/releases)
- [Changelog](https://github.com/nov/rack-oauth2/blob/main/CHANGELOG.md)
- [Commits](nov/rack-oauth2@v1.12.0...v1.21.3)

---
updated-dependencies:
- dependency-name: rack-oauth2
  dependency-type: direct:development
...

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

rspec_api_documentation.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
   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-oauth2", "~> 1.12.0"
+  s.add_development_dependency "rack-oauth2", "~> 1.21.3"
   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"