Update rack requirement from ~> 2.0 to ~> 3.0

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@2.0.0...v3.0.8)

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

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

signalwire.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler', '~> 2.1'
   spec.add_development_dependency 'bundler-audit', '~> 0.6'
   spec.add_development_dependency 'guard-rspec', '~> 4.7'
-  spec.add_development_dependency 'rack', '~> 2.0'
+  spec.add_development_dependency 'rack', '~> 3.0'
   spec.add_development_dependency 'rake', '~> 13.0'
   spec.add_development_dependency 'rdoc', '~> 6.1'
   spec.add_development_dependency 'rspec', '~> 3.0'