Update rack requirement from ~> 1.0 to ~> 3.1

Updates the requirements on [rack](https://github.com/rack/rack) to permit the latest version.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@1.0...v3.1.4)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: direct:production
...

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

Gemfile

@@ -10,7 +10,7 @@ gem 'minitest-reporters'
 gem 'simplecov'
 gem 'simplecov-lcov'
 gem 'sinatra', '~> 1.0'
-gem 'rack', '~> 1.0'
+gem 'rack', '~> 3.1'
 gem 'rack-accept'
 gem 'rack-test'
 gem 'webrick'