Update rack requirement from ~> 2 to >= 2, < 4 in /ruby (#181)

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...3.0.0)

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

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ruby/umbra-rb.gemspec

@@ -39,6 +39,6 @@ Gem::Specification.new do |spec|
   spec.add_dependency "concurrent-ruby", "~> 1.1"
   spec.add_dependency "redis", "~> 4.1"
   spec.add_dependency "google-protobuf", "~> 3"
-  spec.add_dependency "rack", "~> 2"
+  spec.add_dependency "rack", ">= 2", "< 4"
   spec.add_dependency "zeitwerk", "~> 2"
 end