Update rack-test requirement from ~> 1.1 to ~> 2.1

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@v1.1.0...v2.1.0)

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

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

Author: dependabot[bot]

split-api.gemspec

@@ -21,6 +21,6 @@ Gem::Specification.new do |gem|
 
   gem.add_development_dependency 'bundler'
   gem.add_development_dependency 'rspec',       '~> 3.11.0'
-  gem.add_development_dependency 'rack-test',   '~> 1.1'
+  gem.add_development_dependency 'rack-test',   '~> 2.1'
   gem.add_development_dependency 'rake',        '~> 13.0'
 end