Update rack requirement from >= 1.6, < 3.0 to >= 1.6, < 4.0 (#56)

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@1.6.0...3.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

kitchen-vra.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "test-kitchen"
   spec.add_dependency "vmware-vra", "~> 3.0", ">= 3.2.0" # 3.0 required for vRA 8.x
   spec.add_dependency "highline"
-  spec.add_dependency "rack", ">= 1.6", "< 3.0"
+  spec.add_dependency "rack", ">= 1.6", "< 4.0"
   spec.add_dependency "ffi-yajl", ">= 2.2.3", "< 2.5.0"
   spec.add_development_dependency "rake",      "~> 13.0"
   spec.add_development_dependency "rspec",     "~> 3.2"