Update rack requirement from < 3 to < 4

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@0.1...v3.2.4)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 3.2.4
  dependency-type: direct:development
...

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

Author: dependabot[bot]

resque-scheduler.gemspec

@@ -48,7 +48,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'minitest'
   spec.add_development_dependency 'mocha'
   spec.add_development_dependency 'pry'
-  spec.add_development_dependency 'rack', '< 3'
+  spec.add_development_dependency 'rack', '< 4'
   spec.add_development_dependency 'rack-test'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'simplecov'